Since I didn’t see any responses that directly answered the question of what do you DO, I’ve prepared a short guide for a generic social gathering. This guide may be inappropriate in some contexts such as a dinner party or event/tv show watching party, etcetera:

1. Show up

• Not at the exact start time, but at a minimum of 15-20 minutes “late”
• Bringing an unopened bottle of wine or a 6-pack of beer, or another drink of choice is almost always a classy move.
• If you drove, don’t park like an asshole. Consider the neighbors.
• If you’re standing on the doorstep and you can hear music, it’s probably safe to just walk in (make sure you’re at the right place!). Otherwise, knock/ring doorbell.

2. Party!

• Get yourself a drink and/or a plate of food if snacks are out
• Find friends and say hi! You should probably know at least one other person. How else would you have been invited?
• Explore! Hosts expect people in their house so it’s generally okay to look around, admire artwork, investigate the music, go into the backyard, etc. Don’t go anywhere that’s obviously closed off, unlit, or otherwise not a party locale.
• Talk and socialize. Meet new people! Ask your friend(s) to introduce you to their friend(s). Lightly eavesdrop on convos for something interesting you can talk about. Listen in general. Ask people what they like to do. Share stories about yourself! Pro tip: the length of your stories should be proportional to how well you know the person you’re telling it to. Just met the person? Suuuuper short stories. “No way! That reminds me of the time my cat was in the bathroom when she got hit by a car! The vet said she was very lucky to have a good friend!” (Confession: I used autocomplete to write that story)
• Dance!
• Find the host and complement their place, the party, the music, food, whatever. Just be nice. Offer to help if they look at all stressed.
• Play party games like beer pong or whatever.

3. Leave! Say thanks. Take your stuff. Cleanup whatever plates/glasses you’ve used. Don’t drive drunk. Don’t overstay your welcome.

This was probably too much info. I have insomnia. I hope someone reads this.

(Sorry I kinda alternate between second and first person, between recounting experience of what I do and giving advice, may make for a weird read so apologies in advance)

For me house party or bar it's usually:

• arrive roughly half an hour to an hour late. (If it's a close friend's party however I'll arrive early or on time to help set up and kick the party off right, decent friend maybe 15 mins late)
• try to find the host and say hi, if they're occupied talking to someone else, then no worries, give them a wave and hopefully circle back at a better time (if the host knows you've arrived, and they're a good host, they may help introduce you to people later e.g. they say to whoever they're currently talking to "Oh my mate luklmy is here, he also has a beagle, I'll introduce you!". This gives them a chance to catch up with you, introduce you to someone new, and then excuse themselves to work the room
• a good opener is "Hey I'm YYYY, how do you know [host's name]?" it gives you a bit of an insight into the person (are they a school mate? Diving buddy? Valorant squad member?) and your first springboard into a conversation. If they don't have a good thread to pull at, then it'll turn to you to share how you know the host. It can be good to roll into anecdotes from here e.g. "[Host name] was such a brat at school, did she tell you the time that..." or "[Host name] is a beast at Valorant but she wouldn't let on, last night it was 3v1...". That'll give you the threads to continue
• it can also be good, when appropriate, to ask "Hey do you know anyone else at this party?" and give them a chance to point out more people, or call them over into the conversation, now rather than a one on one convo, you'll have a bit of a group convo going on. You may be involved (great! You're meeting more people) or you may get kind of closed out as they talk about in group stuff. If the latter then no worries, you can make a clean exit by saying "Hey it was nice talking to you all, I'm gunna go say hi to those people over there" you could even double back to see if the host is free
• if a conversation is going well and we're vibing, I may say something like "Hey can I get you a drink? Do you know where the esky/fridge is?" or "I'll go grab us some chips. Which way is the kitchen?" or whatever then make my way over. With any luck there may be someone at the esky who I can strike up a convo with, and if they also seem cool then say "Hey come over and meet [person you were just talking to]". The person you were just talking too as well may have started talking to someone new in the time you were going to get a drink, so you've got another person to meet straight when you come back! Too easy!
• if a conversation is getting stale or going poorly you can break it off in a few ways. Similar to the above you can say "Hey it's been really nice talking to you, I'm gunna go say hi to person over there/get myself a refill/check up on the barbecue/dip my toes in the pool/find the bathroom" then go do that thing (you don't want to be the person who ends a conversation, then walks around the corner to stare at their phone, then the person you were talking to comes around and sees you.
• If you smoke, you can take a smoke break. Honestly, this is terrible advice, but I was a social smoker because it gave me a ready excuse to break from a noisy party. You've also got a ready made group of people to meet. Even if you don't smoke, you can go hang out with them with the line "Hey I'm just gunna go get some fresh air" then make your way outside.

There's a bit of ebb and flow at the start of the party as the vibes settle in. As you probably noticed there's a lot of making polite excuses to move around the party or draw more people into your convo if you prefer that. Once convos get big enough and people have put away a few drinks you may get people suggesting drinking games or Smash Bros or a dip in the pool or whatever it may be. It's fun to get involved.

For me, if I'm drinking, I may hit a point of being buzzed enough that it feels like second nature, I become a lot less self conscious, and enough stuff is happening around me to react to that things don't require effort to get going.

If however I've had a few drinks and things don't take off, and everything still feels stilted despite my best efforts, I'll probably just call it a night. I'll find the host if I can and make an excuse (Have to feed the cat/last train home/visiting my parents tomorrow) if it's a good friend I may say as much "Hey man just not feeling it this evening, gunna make an early mark, thanks for throwing the party". On the way out, if there was anyone I particularly enjoyed talking to, I'll make sure to say goodbye and if I haven't already I'll ask for their contact.

As dasharezone says, if you're not feeling it you can just hit the bricks.

Hope this makes sense, it's kind of a base format that is essential to most kinds of standing party, alcohol or none, daytime or night.

The reason I like lemmy, and reddit before, is that it feels like a cocktail party. You wander into a conversation already in progress, listen awhile, maybe say something, maybe make a new human connection.

You basically hide in the kitchen with the other kitchen hiding people until the party ends, particularly when someone in the living room picks up an acoustic guitar.

From an autistic person's perspective who generally enjoys parties: (autistic person with both extroversion and social anxiety) The main goal is to establish and strengthen social relationships. There are a few other goals and notes the support this goal, but if you do not wish to do this, then do not go to the party.

Secondary Goals:

• Survive. Typically, you don't want to come away from a party feeling like its damaged either your emotional state or relationships. If this means taking a noise break so you don't melt down in the middle of the party, so be it. Phrases like "I'm just taking a sound break, it feels kinda oppressive in there." if anyone questions it. This doesn't have to be music, it can be people's voices reflecting off walls as people try to talk over each other. If there's a drinking game and you're asking this question, try not to win or lose. Be in the middle.
• Vibe. At some stage or another, it entered into my head that reading other people's social needs and responding to them was somehow manipulative. Maybe it is or isn't, but try to pay attention to the energy of the room. If people are talking about a board game or their kids first day at school, don't drop your hot take about Palestine. There are definitely parties where that conversation is the go, though.
• Push medium and long term responsibilities from your mind. For the next few hours, you don't have taxes or homework unless someone wants to commiserate about them with you. Commit an amount of time afterwards to unwinding as well (or having a hangover). You and everyone else here is trying to use their leisure time.

After that, there's a bunch of party specific activities that serve these goals:

• Conversation: Obviously. Hypothetically, you might have a standard group of friends that you hang out with on discord and play video games with. While it's fine to chit chat with them, this is not super different to voice chat. Try to engage with a new person or two, move between groups. Typically, people will gravitate towards the more interesting, outgoing, and/or hotter people at the party. This is fine for you to do as well, but be aware of whether you're monopolising someone's time, or they clearly want to end the conversation. If you have a special interest, try to talk about something else unless people want to know about it.
• Drinking: This is a cultural thing, but drinking in moderation can lower social anxiety a bit. Depending on your experience with drinking by yourself, bring your own drinks. You don't want to get too drunk (risky, embarrassing), and you don't want to be in a corner sipping drinks all night (some of the night is fine). Be wary of anyone who wants to get you drunk (especially you specifically). Also, watch your drinks.
• Dancing: Not my wheelhouse, but in some cultures its extremely weird if you don't dance. I don't have any advice here.
• Others: Board games, bad movies, asshole playing guitar in the living room etc. These are all fine and normal, but remember they should be bouncing off points for chill conversation. Don't be too competitive or whatever.

Ideally, after a party, you'll have a few new people to talk to regularly, and may get invited back to future parties or other social functions (hiking, video game groups, a roleplaying game etc). Don't jump down anyone's throat after the party with conversation unless they were really really into you. They probably are not.

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You have a drink in your hand, you go “oh hey it’s Alex how have you been man?”

Alex tells you about his new hobby.

You gesture to your friend “hey Alex have you met Jamie?”

Alex and Jamie get to know each other.

Rinse and repeat.

Jamie meets a friend and starts chatting. You awkwardly stand beside Jamie taking sips of your beer until Jamie says “hey Andrea have you met my friend 420?”

You introduce yourself.

After a while everyone has met and had a few drinks and some good music starts playing and you start dancing in the dance floor. At first it’s very self conscious but after several songs and another drink you start vibing.

If you've ever been to a festival or fair, think of it as a miniature version of that. It's mostly just a hangout for people to do fun things they wouldn't do everyday, since they feel the day is exceptional.

Mostly eat, chat, play videogames or board games really.

And then I show up.

Look at my watch and wonder when it's socially acceptable to leave.

Talk for 2-3 hours until dark. Drink, smoke weed, and then spend the rest of the night listening to music and talking.

Drink, get hit on by girls that want to know why I'm so quiet.