If he isn't neutered that's most definitly the problem! When Hershey wasn't neutered when he was younger he used to bite me and break the skin. I would pick him up look him right in the face and say "NO!" then set him down, usually then he start to scratch the couch, so when he tryed to bite my finger I shoved it in his mouth and made him gag,he couldn't chomp down either,then I would push him back, he'd try and try again he really was a little bugger but eventually he learned. You could also scream out really loud when he bites you to show him he really hurt you(even if he really didn't). Hershey is very friendly now,especially since he's neutered,sometimes he puts his mouth over my finger and lightly puts his teeth on me to say "move outta my way mom!". I got a new rabbit,un-spayed female, she bites ALOT,her favorite thing is biting it seems, the other day she bit the top of my hand, again, I picked her up,looked her right in the face and shouted "NO!" and plunked her back into her cage. A while ago we took her to the vet for a checkup and my sister was holding her and she bit her so hard on the finger right beside the nail and we even heard a little crunch noise(yuck!).
Does your rabbit bite you also? Maybe he feels threatened by your friends! Frightened rabbits bite and they bite hard! Tell your friends to put their hand in a fist,let the rabbit sniff you, then lightly pet your rabbit on the back or side, give your friends a treat to give to him too,food is always the key. Well good luck!