E.P.
Uggghhh, this stuff isn't supposed to happen in kindergarten!!! That's sad. I know in our school, the teachers help by trying to encourage the parents to invite all same-sex kids to a birthday party so that others aren't excluded. Because your child is young, maybe it is an oversight by the parent. I would definitely tell the teacher to make them aware of this. Especially since your daughter is being teased. That's just mean. Unfortunately, it gets even worse by 2nd and 3rd grade.
Also, it is one of those life lessons that you teach your daughter - all life-lessons aren't always pleasant. Say something like ...."Wow, I'm sorry that you weren't invited to the party. It sounds like she could only have so many (a few friends...?) people there. That just isn't nice that they are teasing you about this. I hope that you would NEVER do that to any of the other girls in your class when you get invited and they don't." Maybe tell her of an incident that happened to you at a young age that she can relate to this story. It's hard as a parent to see this happen too. Been there, done that. It will have a positive outcome if you handle it well and nonchalently. Good luck.