Don't feel you have to play all the time, only when you have something to add. Much worse than guitar players strumming in unison is several picking when not exactly doing the same pattern in time.

Most importantly follow the person doing the song don't lead them. It really pisses me off when I hear someone start off a song or tune and another player who may, or may not, be more accomplished then drives it along with a different tempo, pattern or even just emphasis. Let people do their stuff their own way and remember that if you are joining in with what someone else is doing you are backing them.