Jamming isnt something you can learn that quickly, really.
It takes time, and even then some people have problems with it.
Know your role. Sit back, lock in with the drums, and be tight. Dont try anything too fancy, unless you're 100% confident and comfortable. Obviously, you need to watch what others are playing, and go on from there. Keep an eye on the guitarist, especially, and just keep the root of whatever chord he/she is playing. Jams tend to be lead by one person, and then everyone else does their bit. There maybe a "leader" type that starts off a lot of jams, or seems to dominate them. Keep an eye on what they are doing, until you get comfortable enough to add some magic.
If you're starting a groove, keep it simple.
Dont think too much. Relax. Just try and go with the flow of things...
Good luck with it, and tell us how it goes!
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