I've been in a jam night house band for the last 12 years. It's a gig on a week night so all is good.
You meet all sorts. Some jammers are clueless, some are careless, some are stupid, and many are way too loud, but most are fine. Just keep an eye on them and don't let things get out of hand. It's your gear so always be in control.
We are a loud band and the jammers tend to try and match our volume, so I bring lots of speakers so they can't damage my gear. I still keep an eye on what they do and have turned them down mid song if they are being silly. I never lend my bass, ever. If they haven't bought a bass they either borrow one off another jammer or they don't play.
Most jams I've seen start with a couple of numbers from the house band and from then on you're playing all night and only get a rest if another bass player turns up. We usually get one or two. You'd be surprised what players expect you to busk/know. Some numbers are a train wreck.
Many jammers are out of their comfort zone and under a degree of stress so I just smile and look after them as well as I can and be as positive as I can.