Plan B who?
Nobody wants to be anybody's Plan B. Everybody loves to be someone's Plan A simply because it means you don't take people for granted. Just because some people are always available, that doesn't mean you can put them at the bottom list of the people you PLAN to hang out with.. which also means the last options when anybody else is not available, you know.. the losers. Why not inviting all and let's see who can show up? That would be less hurtful, no?
If only you would know how it feels, you won't treat people that way.. ever.
Don't go around Plan B'ing people, unless you are Ben Cumberbatch or Chris Evans or Cara Delevigne, then people would die to be on your hangout list, even if it is at the bottomest ^^;;