that most people in youth communist groups are there because they want to belong somewhere.

I don't know if that's the primary reason, but it's certainly valid to want to belong. It's why people join gangs, terrorist organizations, etc. Perhaps it also applies to commie groups, we shouldn't think of them as holy mystical orgs free from real life issues. I get that your friends have no idea what they are talking about, but I do think that people join orgs not solely for helping others, but also helping themselves ('it's better to be together with like-minded people than to suffer alone' king of thing).