It's quite easy....
Put an exotic on the line, there are two halves to it.
I enter raid matchmaking and put any exotic weapon/gear on the table, if I quit/lose connection/whatever, I lose a half. I still have the other half to use at another time and if I use it and quit/lose connection/whatever I lose the exotic.
-OR-
Put coins on the table
Either way, make a wager system. I want to raid but am not "social" enough to do so.