L.R.
Was it like this last year or in earlier years in school? Is she usually shy in other situations or only at school? Is she, overall, quiet and reserved or is this unusual for her? Is she involved in any activities outside school (Girl Scouts, sports, dance, art, any kind of classes or clubs) and at those, is she "odd girl out" or is she socially comfortable? When she has these girls over to play at home, is all OK and they're all interacting well with her and she's interacting happily with them in that setting?
I think you need to examine the whole of her life, not just her school life, and consider whether this is a factor of her own fundamental personality or something specific to her school, these friends, this year. I'd sit down and really think hard about what you've observed in all those places and situations outside school. If she seems to have this issue only at school, is it only at recess or is it throughout the day? Kids have many other interactions besides recess where they are being social (lunch, going to and from classes, interacting in class time, etc.).
Have you talked with her about whether anything has happened? There may be no specific break with these girls; they may all be girls who have other connections outside school (live near each other and have known each other forever; in the same dance class/GS troop/sports team/whatever outside school; love the same pop star for whom your daughter cares nothing --whatever). Or there may have been some specific issue that you don't know anything about. You need to get her to open up to you.
Another aspect: Is she the kind of kid who is willing to walk up to a group and insert herself, or is she possibly hanging back waiting for an invitation to be issued? This came up on here a while ago about a girl who seemed to feel she needed to be specifically invited to play, when really she could have gotten up and gone and joined a group. When you say these kids "exclude" your daughter at recess, what exactly does "exclude" mean? Does she say they do that? Do you think she could be using that term when really they might be happy to include her if she were more assertive and walked up and said, "Hey, I'd like to play X too"? Or has she tried that and they actually and clearly said "No" to her?
A lot of questions here but I think you do need to do a lot of exploring with your daughter and a lot of thinking about her personality and her social abilities both at school and outside school. If she is shy that is a larger issue and she may need some help to get more assertive and not wait to be asked to play; if she is trying to play and being flatly told no or ignored, that is another issue too and she needs some tools from you to handle that. But it seems odd that girls who play at your house OK would then do flat-out exclusion on the playground -- doesn't it?
She's hurting but you can help her. Rather than asking what you can do to help -- which may stump her, frankly, because she knows you can't really help then and there on the playground -- talk to her about what exactly happens when they "exclude" her and whether she could approach them more, or find other people to play with. They're not the only ones on the playground. And she can play by herself rather than sitting there -- just moving around will put her in other kids' orbits and start impromptu "together" play eventually.