It's assault.
You need to get your dd to understand that this is 'hazing' behaviour, establishing how far the group can go with her. Basically, if she tolerates this, worse will follow.
My bet is that there were other, smaller, incidents prior to this, that she let go in order to keep her place in the group. And now this has happened.
She should get tested. Given their ages, it's probably going to be OK.
She should definitely leave the group. Given that she's prepared to put up with a lot to stay in it, I guess that will be hard. But she needs to get her head around the fact she's at the bottom of a nasty group and things are only going to get worse from here on in.
You can try with the police. I think it's the right thing to do. Whether much will come of it, I don't know. But it could at least separate her from them.
How incredibly awful, though. I'm really sorry for you - that's a really shocking thing to have happened.
And it is an appalling thing to happen to your daughter. Really appalling.
Don't minimise it.
I've two older children and nothing like this has ever happened to either. Both would be horrified to think of this happening to a friend - and certainly not by 'friends'.