If you can read the lead, life is good. It's nice to know the song, and a good idea to ask the lead performer if s/he's adapted/arranged the usual melody or chord changes. Some people arrange songs so that they retain the 'flovour' of the original yet become a new song in the process. For example, if a song goes G,G,G,G,C,C,C,C and the lead has changed the chording to G,G,G,G,Am7,Am6,Am,C, ya might want to sit out the first stanza and come in on the second instead. And then there's what Kendall said. No one has to play on ever song.

BM