Every group is different, so your best bet is to get there early and ask the leader about how this group operates. In particular, I would want to know if the group rides in a tight paceline and if there is a "no drop" policy. If it's a ride specifically geared for new riders, the answer to the first question to be "no" and the second one to be "yes." I'd also want to know what speed the group hopes to average. Again, if it's a ride for newbies, I would expect that to be pretty reasonable. In general, I would expect your ride to be more social than anything.
In general though, I would suggest that you mostly just use common sense. I like to stay to the rear when I'm new to a group and just watch how it operates. If other are riding behind me, I would use hand and verbal signals to a indicate turns, road hazards and the like. Just ride predictably-presumably like you normally would.
I would also to encourage you to arrive on time and to come prepared. Have enough water and food for the ride. Have the gear necessary to change a flat. Make sure your bike is in working order. If the ride doesn't have a no drop policy, make sure you know how to get back home.
I like to introduce myself as much as I can when I'm new to a group, too.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher