Prison Pit Team Building exercise?
I hope they don't institute that at MY workplace!
My previous DM said repeatedly he wanted to use the "all in prison, work together" approach.
My son's got a group of friends who always start with a session where the players and GM talk about what they want to do, what the world is like, etc. They eventually decide on a coherent reason their characters are together, creating back stories at the same time. I've never done that, but I can see advantages.
For my current (RM2) campaign, which is explicitly designed to be a series of adventures with no grand world-goals, I'm trying something different. I've got 6 players/characters, each one chose their character from any part of the continent and any culture they wanted. Once I knew the characters, I came up with "cosmic mystery loaner powers" for lack of a better term. Each character will gain some minor benefit from another, but only after 24 hours together, and it fades if they're more than 100' apart for 24 hours. So, for example:
Rogue from multicultural city grants monk from far off 3 ranks in speaking each of 3 local tongues
Monk grants bard extra power during battles
Bard (half-orc) grants wood elf 1x healing rates, instead of normal 0.7 for wood elf
Wood elf cleric grants nightblade +5 DB, +10 versus shared cultural enemy
Nightblade (with a familiar with excellent hearing) grants fighter excellent hearing
Fighter grants rogue +2 ranks Ambush
They've only just met, and a couple of them have noticed their changed powers. They haven't figured out where they came from, but I think the fighter suspects.
I doubt this will ultimately matter for them staying together, I think they're all experienced enough players to do that. But it will add a mystery component to "hey, why ARE we hanging around together?"