I have 3 girls, the little one is 5. I am afraid girls do this to eachother.I love having girls and the upsides far outweigh the down sides.A positive spin on why girls do this is that they, from their earliest moments, are highly relational and seeking deeper friendships.However as life is a steep learning curve, they do it badly and to the exclusion of others at times.
I have encountered such behaviour even at nursery level - "You're not my friend anymore", "You can't come to my party","You can't play with us because we are the ice skating club" (really!) It hurts but I sometimes wonder if it hurts us more.I would, as far as is possible, play it down with your daughter.I say things like "oh well, they are just having a bit of a grumpy day/week.Find someone else to play with and if you can't ask the lunchtime supervisor/Nursery Nurse etc to help you find a game to play".Keep bright and light about it and her feelings will not be as hurt ,if she does not receive the signal it is a big deal.
If it continues to happen then make a concerted effort to have some other girls around to play and don't be scared to mention it in a low key way to the staff.They can tackle this in circle time in a non heavy way.
These issues don't go away for a long time.My eldest who is coming up for 15 has had the most settled 2 years (friendship wise) that she has had.They do seem to grow out of it to a large extent.
My middle one was recently told by her 2 closest friends " We do like you but we don't want to be a three anymore.We want space to be a two and you can't share with us on the school holiday.....We still like you though"(Oh well that's o.k then
It's hard, I really know that ,but especially at this age, don't get too emotionally involved in it all because there is a strong chance they will all be playing happily again by next week!
Keep an eye but I bet she still had a lovely morning - it is us mums that carry these things in our hearts when the children have often moved on.
Good luck with it.