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.
Ok, I've reposted it in the other community. Feel free to nuke this one!