Been running a blues jam for around 5 years now (as well as housing at other jams).
Basically You provide the backline and depending on the etiquette of the jam a cheap, cheap bass for anyone who turns up to use (I take a £100 special with me, no one touches the others).
The house band start the night with a few numbers to warm the room up, and then I put 'bands' together from whoever has put their name down on the list. Some nights can be manic and some very quiet. The drummer and I have been known to play three hours without a break on some nights.
If a punter starts getting over loud...turn them down...if they complain turf them off.
Keep good eye contact with the drummer, there is no one better to shorten a 15 minute slow blues.
3 questions you really need for anyone getting up:
Straight 12
Slow 12
shuffle
then go 1, 4, 5.....if it's anything different the leader of the 'band' should tell you (although I've played with some leaders who think I'm psychic), or just ask for the basic changes.
It can be great fun, and a# good way to learn and build your repertoire, and don't worry if it all goes to sh*t...it's a jam.