Overall, it's very nicely done, IMHO. A few specifics:

Your dodge around facebook's "no more than 10 invites at a time" is clever.

That said, your app has the good fortune of being inherently viral -- users that want to collaborate with friends will invite those friends. As such, I would recommend toning down the requests to users to invite their friends. It comes off as being too pushy (to me).

It's quite plain-looking, which is normal for a word-processing application but out of place for a Facebook application. I would err on the side of fitting in with your context -- add more icons next to functions, etc.

Congrats and good luck!