Having the friends over is an opportunity. I walked in on a bunch of teenage boys using chat roulette for a "laugh". As a group it probably was harmless, had they done this alone they could possibly got in to real trouble. I told them that girl could easily be a 60 year old fat man, it could also be a really young girl, either way it could get you in trouble and not what you are looking for. They were genuinely repulsed and I'm pretty sure they got the idea..
Do any of their friends use internet filtering? Probably not anyway. They may well not even know such sites exist.
I'll consider putting together a having the friends over AUP with choices. To cover internet use, games ratings, healthy food, taking of photos, use of suncream, health screening, allergies, screen time and bed times. That'll go down real well with their friends and make me look like a nut case.
Supervision+Trust goes a long way. I do filter my 8 year old,