Sublet to a third roommate short term?

[Amy Wirsching]

Out potential new tenants just sent me the following email: "We're considering a third roommate - a female adult without children. She wants to take a look at the house as well. Our preference is that Sam & I be the lease holders and that we be given written permission to sublet as our friend will probably only stay temporarily. We'd take on full responsibility for her use of the property. AND - I understand if that's not agreeable. If it's not agreeable - and she must sign the primary lease - I'd just like to have written agreement that she could vacate the property at any time and Sam & I would remain on the lease and complete our term."

I am open to the idea and would complete a background/credit check on the third person. What would be the best way to implement this arrangement? Anyone done this and how? My first instinct is to avoid the sublet option.

Thanks!