I agree with Valerie. It doesn't sound like those girls are "good friends" and someone you want your daughter to be like/spend time with going forward.
I realize that may be hard to accept since they live close. You don't want to isolate your daughter from those who live in her community. However, since you asked about encouraging her to stand up for herself, I suggest she walks away from the other girls next time they aren't being nice to her. She doesn't have to say anything - LET THEM PURSUE HER. Then when they do ask (when she's walking away or next time they want her to "play") she can very simply say, "I don't appreciate the way you treat me. I've mentioned it before and nothing has changed. So if you can't be nice and include me in things, then I'm going to find something else to do."
I also agree that you should help find other interests/friends for your daughter. Consider having her try out a new sport or program at school this year (when it starts), like track, theatre, cheerleading, etc. or consider the local YMCA or pack district - they should have TONS of programs going on right now. Let her look through the catalog with you and pick one or two new classes to try. Make it clear that this is FOR HER. So if she doesn't want to do the class again, she doesn't have to. That way she's not feeling pressured by you to "have a hobby".
Hopefully (and most likely) she will find a program or friends that do the things she likes. Make sure you ask her after every class, what did you do, did you have fun, tell me about the other kids in the class. This will help her to open up and meet the other kids so she has something new to tell everytime!
You can show up early a few times (before class ends) to see her do her thing and maybe meet some of the other kids & moms. Then ask if she'd like to have someone come over and play or meet at the library. Be willing to play chauffer.
By putting her in a new situation where she thrives, she'll have more self confidence and realize what REAL friends are instead of being STUCK with the girls in her neighborhood only. The kids in the neighborhood can be fine to play with once in awhile. However with the pattern they are creating, as a Mom, I'd be concerned if that's ALL she had.
I hope you find this helpful! Let us know how it goes.