Any time I'm reading a new RPG and come to the section on skills, the one question that always comes up at some point is "But how does it want to handle the whole party sneaking past guards together?"
The most simple approach is that every PC makes a skill check to avoid being seen by all of the guards, and that every guard makes a skill check to notice any one of the PCs. If only one of the guards manages to roll better than only one of PCs, the whole thing is up and the players are discovered.
And when you have four to six PCs and four to six guards, even the PCs are all much better at sneaking than the guards are at noticing, it becomes statistically very rare for the players to ever get past guards unnoticed.
What other approaches to handling such a situation with skill checks have you come across or come up with that makes sneaking past guards with the whole party a more feasible option for the players with a decent chance for success?
Blades in the Dark has a pretty fascinating approach that comes from the unique way is resolves actions in general.
If you sneak as a group, it is done as a group action. Everyone rolls, and the best roll in the whole group is the result for the entire group. Sneaking with several PCs actually makes it more likely to succeed than going at it alone. (Which of course makes more sense with most other actions, though perhaps seems a bit weird with sneaking specifically.) However, as a downside, every PC in the group whose roll is a failure causes the character leading the group to take 1 point of stress. Stress is a pretty unique mechanic in Blades in the Dark (and other Forged in the Dark Game). In these games, players can at any time declare that the bad outcome of a roll is actually not as bad as the dice made it look, and the consequence can be shrugged off and the character is fine. But that always costs you stress. And once a character's stress reaches the limit, things get bad. The character is out for the rest of the adventure and will suffer a permanent mental mark for the rest of the campaign.
For a character who is leading the party in an attempt to sneak past guards, you have a pretty high certainty that you will get past the guards undetected. But he's the one who is probably going to have to pay for that by not being able to avert other disasters later in the adventure.
I haven't used this but while reading the comments it came to mind: what about using something like a 4e skill challenge? Say they need a number of successes before a number of failures (maybe successes is determined by the number of sneakers and failures is determined by the number of watchers somehow, or the skill of the best watcher). Then it's up to the players to justify how they assist in scoring successes. Sneaking, disguising, distracting, etc.