Said as the mom of a seven year old boy who can, at times, be very insistent and very intense~ and said kindly~ do your best to let this go.
First, I see this exclusion with kids all the time in my son's school,even within the same class. I wouldn't say it's meanness inasmuch as it's kids who are exercising their opportunity to make choices-- and you know, as many have pointed out, they just *aren't* graceful with this.
(Think about how many adults you know who aren't always graceful about the choices they make.)
When my son complains about playing alone or exclusion, I remind him that he if wants company, he often will need to play what the other kids are playing. That if he's feeling lonely, look around for that other lonely kid and ask them what *they* want to do. Try to remember, too, that while we adults are polite creatures (well, *more* polite in any case), for a lot of kids, ASKING to play is the death knell of the social interaction. This is because a lot of kids this age are like toddlers--- give them a chance to say no and they will take it, just because they don't get a lot of choice or autonomy at this age. They naturally separate and exclude... at least, this is from what I have seen.
I'm telling you all of this because I've been the heartbroken mom too. But within that heartbreak was what Suz T alluded to-- the sad understanding that the 'herd' was trying to communicate something which I could not. Over time, I've stopped getting so emotionally involved on this and just given him lots of positive encouragement and pointed out who we know *does* enjoy him. We focus on the friends who do play well with him -- maybe they don't want to play all of the time, but part of the work of this age is learning how to share friends, how to be a friend and youngsters are still figuring out what that looks like.
The messages from the herd are so important for kids. Our son has some minor delays as well some deeper disorders we are addressing. My job as a mom is to make sure that, even as hard as it hurts to watch kids snub him when he wants to play *only* his game-- I have to make sure that, over time, my son is able to learn how to pick up on social cues and function as a part of a group of people. Which means I can't shield him from the little stuff. Of course, we deal with more serious situations (like when he was being physically hurt by another kid last year), overall, though, I know that his desire to be part of the group will be such a great tool in helping him to modify his own actions that I don't want to get in the way of that, no matter how hard it is to watch. And it IS getting better-- the more I back off, the more he learns. Truly, have faith-- your girl is still very young.