Like yourself, Ive been really focusing on improving my playing and theory also(played for over twenty years and have recently started playing with new groups), and my advice from that would be to combine what both these wise gents have offered, along with trying to not feel overwhelmed, there is usually at least one guy in the group who will tell you if youre stinking up the joint lol trust me I know !. Sounds like you are fitting in well so far. If you dont have the opportunity to spend time with the keyboardist, and you have basic music theory, I would suggest listening to as much music as you can if you arent too familiar with the styles. Jazz voicings as was mentioned are used alot, extended voicings 7's 9's 11's 13's, altered chords,, using secondary dominants, chord substitutions, diminished and other passing chords, cadences, jazz progressions , etc etc etc. Listen to the embellishments other players use, how the guitarist sits in, on or out of the mix and groove. Just some quick random thoughts, hope one of them helps.