Watch the Continuity online guitar lesson by Robben Ford from Rhythm Revolution: Vamps & Jams
This is a nice little vamp between G and C, somewhat inspired by The Meters. You always want to be paying attention to what's going on in the bass and drums as the rhythm guitarist. They carry a lot of the rhythm, but you do too, and you'll be carrying the harmony as well. So, listen to the bassline and figure out where you can fill in the rhythm section. You want to allow room for each of the instruments to speak.
I personally play a lot when I play rhythm guitar: I like to be adding rhythmically and harmonically to the whole thing. But, even that being the case, I'm always referencing what's going on with the other instruments. You should always consider yourself last in the rhythm section. Let everyone else do their thing, and you add. The more experienced you are with playing in rhythm sections, the more you'll be able to play.