Jamming in my neck of the woods is being able to wing it when someone says; "The next one is kiss ole Kate, let's do it in D, ready one and two and.............."
You've never heard "Kiss ole Kate" so..... you assume some stuff - roots right at first. After several measures sneak in a R-5, perhaps a R-5-8-5 with I IV V I progression in D. Change chords when the rhythm guitar guy does. Yes basic 12 bar blues progression is safe and if you get lost do a I-V-I vamp till you catch up and if everything fails and you are completely lost
build a groove with the tonic pentatonic. As long as you are using in key notes and keeping the beat you are functioning in a jamm session.
Have fun, keep doing what you'll are doing and add to your gig book. Play from chord charts, chord charts (fake chord sheet music) will let you play hundreds of songs.
Google can find them for you. Google ----- Guitar chords, "name of the song" the comma and quote marks help. For example; Guitar chords, "Cold, cold heart". gave me several choices I liked this the best.
http://www.chordie.com/chord.pere/ww...dColdHeart.cpm
Have fun.