Just say no, not right now, call back later and keep telling them that until your open to it.

What kind of "close" person would ask? Not one that's close at all.

If we were close I'd know what you've put into it and wait for an invite which I may or may not accept. If I thought the relationship would suffer one way or the other I'd do what affected it the least. That's what it means to be close to someone, they matter most.