New idea: Greeting as gatekeeping. 

If you see someone you don't know at a meetup, don't ignore them. First, check if someone is already talking with them, if they are you don't have to do anything. If they're alone, go over talk to them and find out if they're a newfriend from the thread or a rando. If they're a rando, kick them. 

Plus if they're a rando before you kick you can ask who's their friend and learn whos anchoring the randos.

This would eliminate 95% of the randos.