I think you need to spend more time with her. I'm sure she's a very nice person...I'd take her the more the merrier. She can talk to the others too, you know. Tell that she can make more friends while she out with you guys. Then she'll be their problem. You need her to get involved with more people. You're her only friend...Treat her like one, you'll find that you can get the best friend you need for the rest of your life. It's happened to my dad. I swear just try. She loves your company, make her feel needed and she'll do anything for you
But that's just my suggestion