When I do jam with people it's largely familiarity with what 'goes' with certain chords and progressions that helps me.
I don't think I can recognise intervals and chords that easily, but as has been mentioned above, I watch what the other folks are doing and try to remember that a jam isn't about showing off how good you are, it's about playing together, so if you're keeping it simple but still taking part you're still jamming. Nowt wrong with roots and simple stuff whist you get into what's going on.
Also I'm never afraid to ask what people are playing either. It's not a competition, it's collaboration.