Good luck

Im missing a club run today as the family wants to go to the Tissington Trail for a pootle so i might get a swift ride in later
You might find this helpful
http://www.redhillcc.co.uk/page/group-riding
As other have said just go along have fun listen and learn as they should give you plenty of encouragement and advice, my club has a "slow" and "medium" group that have a no drop policy where we try to ride at the pace of the slowest person to turn up and make sure we regroup at the top of hills etc and if someone has a mechanical the group will stop and someone will help.
The fast group is similar but after the cake stop they tend to put the hammer down and its your job to hang on but if your in the fast group then you know what to expect ( mind you Adrian Timmis ex pro sometimes rides on this ) .
I reckon you can easily add 1-2 mph to your average with the drafting effect but only ride as close as you feel comfy , ride slightly to one side so if you do have to stop then your not going to hit the back wheel/ rear mech of they rider in front unless your really confident and have faith in the rider in front.
Listen out for calls /points for holes and hazards .
Have fun , eat cake !