I need Rhett-style advice on this one.

So, there's this girl in my grade, and she really doesn't have any friends. I naturally felt really sympathetic for the poor girl. I tried just being nice by inviting her over (Because it was my understanding that she never got invited anywhere) but now she thinks that we're the best of friends, and she's gotten SOOOO clingy. How do I get her to back off while still being nice??? Help.