Quick background as I can't reply with what I would say if had approached this girl during day game etc. as she is is one of my friend groups.

I am in highschool. This girl started to like me at the start of the school year. She invited me out with her friends and I got to know some of them quite well. However, I didn't really like her at this moment in time. So over the next few months a new friend group formed with me and some of my friends and her and her friends. I started to like her around the very start of 2015. I quickly invited her out somewhere as I thought it may have been too late and she had lost feelings for me(the date went alright). But after that I invited her out again etc. and she was making excuses. I would have normally moved on after that. However, as she liked me for several months I didn't think she could lose all attraction. So last weekend at a party I decided to escalate to see if she would be receptive. She was not. I have started to move on. However I would say I have a bad case of oneitis right now.

She just messaged me saying "I don't want to upset you but do you think we could just be friends?"

How do I reply to this? I can't use something like 'I have enough friends' or something like that as we are in the same friend group now and we go out as a group regularly.

Any ideas? I literally have no idea what to say.

EDIT: spelling