like everyone else said, just tell them your new to smoking and just remember you can always sit out a hit or two to see how your feelin and you could always join back in, theyll understand if you stop a long time before they do. also remember if its their weed NEVER tell them to stop. i hate that.
good luck, youll be fine besides THEY invited YOU to smoke with them so they must think your cool and youll look even cooler once you smoke with them.
first time is usually they best time, enjoy it :jointsmile: