I agree with konapoppy. When DD was dropped from her friend's guest list it was a shock because I had been hosting this friend as part of the neighborhood group for a couple of years. They still hung out on a regular basis at least once a week. I had no idea that they weren't really "friends".
As time has grown, I am no longer hosting a neighborhood group because they have outgrown it, and it is true that the girls are friendly but not friends. I guess I just didn't know that 2-3 years ago, at 3rd grade. If I had known, I'd probably not hosted as much (which was kind of forced upon me by that mom anyway). I wouldn't have worried myself so much about excluding the other girl and worried about her feelings as much. Not that I would be mean, but several times she was only invited because her brother was invited. The boys are friends.
Anyway, the point is, when the kids aren't as close anymore, it does make things complicated among the adults, and that is something I am trying to figure out. I have distanced myself from the mom, (but she did first), but we are friendly and amicable.