Girl drama is correct. I imagine that by the time you read these posts the scene will have changed. Listen and empathize but never assume that your daughter's feelings about her friends will be the same an hour or a day from now.
My granddaughter frequently complains that "people" are mean to her at school and that so and so who was her best friend last week is now her enemy, etc. She's been doing this since Kindergarten and now she's in the 5th grade. This is just the way kids are.
By the way, I volunteer at school and see first hand the reality of my granddaughter's drama. lol
Invite the girls you'd planned on inviting and include the new friend. With 18 kids everyone will have someone with whom to connect. I wouldn't worry about it.