i would do 2 things. first, make sure you tell everyone else on the ride that it's your first group ride, so they know what they're getting themselves into. second, be the sweeper of the group, meaning be the last person in the group and observe the other riders. if it's not a really fast group, they'll will most likely slow down and wait up for you if you drop back, because most group rides don't drop riders. if you're not sure about this, than ask them if they drop riders. lastly, you can be about a foot off the wheel of the rider in front of you and still use his draft to your advantage, meaning don't get too close to the person in front of you and don't overlap wheels. bottom line, enjoy the ride, and as you get more experience you'll know what to do and what not to do...