Yeah, I get that a lot, too. I have no idea why they'd add me and then not want to play, especially since a lot of content requires two or more people that you can't get from random matchmaking. I mean, I could be in the middle of a strike or a crucible match and if someone invites me, playing with other people is so fun that I'll immediately stop and join their fireteam. The weirdest part about that is most of the time when I invite people, they're just walking around a planet on Patrol, not in a fireteam or anything. You'd think they'd want to do SOMETHING with a friend.