"Do you mind if _____ joins us?" my best friend asks.
Yes, I do. I don't want anyone joining us. I'm not saying I'm a misanthrope, even though I kind of, like, am. I'm just saying that I don't want to share you, I don't want to have to accommodate this person's sense of humor or have to censor myself in any way, and from past experience with this person, that would be a requirement if I don't want to come across as, well, a dick.
Or as a baby who says, "I don't like to share my friends."