This might be a little different from most responses... It must be agonizing to watch. My favorite parenting author, John Rosemond, says that when parents "run interference" in their kids' lives, they hinder them in their development of these social skills. He recommnends, instead of getting in the middle of the troubles with the other girls, letting her learn to handle it herself - or she never will learn to be assertive or how to make friends, or how to avoid hurtful people. He suggests making an occasional comment to get her thinking of how to solve her problems and to point her in the right direction, perhaps something like: "wow, I wouldn't want to play with someone who acts so mean". Perhaps discuss it a bit with the mother of the two girls and invite only the younger one over for one-on-one play. Also, to help her take the focus off herself and her sadness so much, suggest that (at school at least) she find someone who needs a friend, instead of waiting for someone to find her. This is a life skill.
Also, I know people mean well, but it doesn't do children good to over praise them and tell them continuously how wonderful they are. I also get this from John Rosemond. That doesn't build self esteem (ego and self centeredness perhaps, but not true self esteem). Self esteem comes from accomplishment. Instead of focusing on these children as much, you might teach her some new skills and give her things to do and think about, such as making cookies as was suggested, or get a keyboard and a very basic book that you could start teaching her to play the piano with (music expands the brain in so many ways, and you could certainly walk her through the first book or two). You might find a good used one on craigslist or something.
You could find a simple book on cross stitch or embroidery, or even sometimes they have free flyers at craft stores (like Michaels) and find some simple project for her to do. Learning a new skill and creating something would develop true self esteem.
If you're allowed pets where you live you might get her something like a cat (spend time with it before you pick one out to see if it's going to be a match) or guinea pigs are supposed to make great pets, or even gold fish. Teach her to be responsible for it so she can have someone else to focus on and care for. You might sign up for www.freecycle.org in your area and watch for when someone is offering a small fish tank (for free), if money is tight. Right now it is truly a sad time for her, so the best thing to do would be to get her mind off her troubles and develop her personality a little more, then perhaps she wouldn't be so vulnerable and wouldn't be left out as much. Best wishes!