Renting a room to your kids 18 year old friend?

[Derek Tellier]

So posting in off topic as I really don't know how this fits in, just wondering if anyone out there has been in this situation. 
My daughter's friend turns 18 at the end of August, her grandmother has already said once she's 18 she has to leave. Today the grandmother "kicked her out". We're letting her stay here for a couple of days and going to follow up on the legalities of her staying till she's 18 tomorrow, I don't want to put myself in any bind there. 
But at the end of Aug she's 18 and free to leave anyway. I've got plenty of room in the house and already had longer term plans to rent out some rooms in the near future anyway. 
I'm leaning towards offering to let her stay once she's 18. She'd have her own room and share a bathroom with my daughter. She still has this whole year of high school left so trying to figure out if I should charge her rent while she's a HS student or chalk these next few months up to charity. She has a job but won't be getting her license till after she's 18 and of course will still then need to buy a car. 
I want to help her out but don't want to turn my home into a halfway house either. She'll need a place anyway so maybe I just charge something small like $100/month? Would you do a lease on something like this? 

Anyone been there done that or have any advice on it? 
Thanks