A guy I barely know claimed that I was cheating on my boyfriend. What should I do?

I would definitely talk to him and make sure he knows that what he did was wrong and that if he ever does it again there's going to be a huge problem. I'd talk to him one on one first and if that doesn't work then the group thing should work because he needs to know that he can't go around lying and making things up especially when it involved your relationship and your reputation. Make sure to talk to him asap while this is still fresh in peoples minds don't let it get to be to long before you talk with him and make sure he knows how angry you are with him