Pay attention. Be predictable. Don't use the brakes unless it's necessary. Don't be unsafe or act stupid. Watch how other riders (who seem to know what they're doing) ride. They actions should be smooth.

If you are nervous, I'm sure the group won't mind if you stay in the back until you get the hang of things.

Do you know what kind of group ride the ride is? Is it a fast ride, or more of a social ride. One ride I do is a constant rotating pace line for 40 miles. It's fast and people get dropped. The group would rather slow riders stay in the back instead of trying to hang in the pace line. Another one I do is a more social ride. Some go fast, some go at a more moderate pace. We usually do a single pace line where you can pull however long you want. Sometimes the group will split, but we have at least ten points in the 35 mile ride where we regroup... and it's essentially a no drop ride social ride