I have an idea that might help me stay out of trouble on many levels.
What if...
I SELL them my bikes, for say $300 each (They are not expensive racing bikes or anything like that) and have them sign the purchase receipt. I will agree to purchase the bikes back at the end of their stay, for the same amount, less any repairs needed.
If they are stolen, they have paid for them, and it's their problem.
If they have an accident, they are not my bikes.
If they hit someone else, or do something wrong, they are not using my property.
Can you find fault with this?