Yes she wants to join. Fuck yeah or hell yeah are a very enthusiastic way of saying yes. E.g. Do you fancy going for pizza tonight? Hell yeah! (Yes!!!!!! I'd love to go for pizza. I wouldn't want to miss this occasion!)

She probably said "yeah" instead of "yee", but, regardless, it means that yes, she will come to your party. Even though it uses a curse word, it's a way to enthusiastically accept an invitation. The opposite would be "fuck no." Which means that it is an absolute no. You probably know this, but the "f" word is one of the worst/strongest curse words in American English, so only use with close friends who don't mind cursing. :)

I'm guessing "fuck yee" means "fuck yeah" which means yes!

Fuck yeah - yes. Fuck no - no. Fuck that - hell no/never. And yes, like others have said these are all extremely informal. I would suggest only using these with friends, or if you're a student then you can use it with other students you are familiar with. "Hey Anna, there you are. I was wondering, were you going to that class later with the new teacher?" "Lol fuck that." These days it's not uncommon to hear responses like that between students in university who are familiar with each other.