When One is Left Out

So far we have avoided the drama of only one of the girls being invited to a birthday party or play date. At this point, when one is invited, they have all been invited. I'm dreading the day, though, when I have to explain why only one is invited somewhere. (And, just a side note, I completely understand why all four may not be invited to a party... different classes, different friends, etc., but I still think it will be hard to explain).
Still, there are times when my daughters exclude one another from various activities. The other day, for example, Roo and Sue Sue were playing some game that made sense only to them. It involved sitting in an upside down chair and talking about bugs. Tortilla said "Can I play?" They said no. Keep in mind that as soon as Roo said no she looked to see my reaction. She knew she was supposed to say yes. I diverted Tortilla's attention and she went on to play something else.
It's a fairly common occurrence in our house: someone is playing quietly by herself and suddenly her three sisters want in on the action. I'm all for share and share alike, but I also understand their need for some alone time. Sometimes I can split them into different play rooms. Other times I set the oven timer so they can take turns with toys.
Still, I hate the heartbreak on the face of the sister who is told she can't play. It's never the same sister and never for the same reasons. Being left out is a life lesson that is never fun to learn.