You need a little more information than just "E" - minor? major? blues?
However, once you've determined that, and assuming that you're jamming on one chord, then you pretty much want to stick to that, unless everybody sort of morphs into something else. One of the coolest things about jamming is that you can basically do anything you want! However, to quote Peter Parker's Uncle Ben, "With great power comes great responsibility." Jamming also requires LISTENING. If you've set up a groove and someone starts soloing and you (as the bassist) start playing something else or start playing in a different key, then that's not really respectful of the person soloing.
One of the coolest things to do is to play something really simple, and just play it over and over underneath someone's solo. If you (and everyone else) are listening to the soloist, then you can all build together and actually take it somewhere.
However, that's only one way to jam. There's an infinite number of possibilities. It can be good to talk about it afterwards with your bandmates too, though you don't want to overanalyze.
Sorry, kind of jumbled! Have fun!!!
