When jamming
I assume certain things. Most of my jamming will be to old classic Country songs. I bet you would find yourself in this same situation, may not be Country, but you pretty well know what type of songs you will be asked to jamm over. So you know Rock will be a dirt simple I IV V. Blues, grab a 12 bar dominant chord progression and hang on. Whatever is called I bet you have already played this type of music before and have a pretty good idea what's coming.
Back to our jamm. Key called to be G - I assume the chords will be G, C, D7 or I-IV-V7. As old time Country does not stray far from that. If I've never heard the song I'll start off with a root nothing or perhaps R-5 and listen for the rhythm flow and the chord changes. I cheat here as I also play rhythm guitar and can look over and recognize the chord the rhythm guitar is forming. I change chords when he/she does.
If I get lost or can not pick up the chord changes I'll play the tonic pentatonic to the groove the other musicians are following.
Nine times out of ten this will work. Jamming is not exact, close enough works. I'd suggest you assume and play what you think will work. If it does not, lay out, or adjust.
Practice jamming to the backing tracks on the Internet. No one will call out the key so you will have to figure it out. If you need help with that just ask someone will jump on it.
http://www.guitarbackingtrack.com/pl...eric/layla.htm
I hear E.