Never draft closer than you are comfortable with. Especially with people of unknown quantity. I usually start off at about a meter or more until I know the people i'm drafting aren't newbs. 99% of the time your drafting is going to be on the road. You should probably get used to it.
Someone more experienced on friendly group rides can probably show you the ropes and give you some pro-tips. Don't expect any of this on the donut ride.