Good plan to have anyone living there sign the tenancy agreement (I'll avoid the term "lease", which is a term of art unto itself). Having the covenant of all occupants gives you more people to go after if things go sour. For example, if there is only one signatory and 3 "tenants", and they all skip out leaving unpaid rent and damage, you will not likely be able to pursue anyone apart from the one who signed. He might be insolvent, whereas the others might have jobs, resources, etc.
I won't go so far as to say a L/L can never claim against someone not signatory to the tenancy agreement. I can see no reason why a L/L cannot make an oral contract with a second person, under which that second person agrees to be bound by the written agreement. The downside is burden of proof. If you pursue that person, and they deny any agreement with you, and claim they were just a visitor or some such, you'll probably be out of luck.
In B.C., a course of prudence is to get all tenants on the covenant, even minors. I say that because of the effect of s. 3 of our Residential Tenancy Act:
Act applies to tenancy agreement with a minor
3 A person who has not reached 19 years of age may enter into a tenancy agreement or a service agreement, and the agreement and this Act and the regulations are enforceable by and against the person despite section 19 of the Infants Act.
I cannot speak for other jurisdictions, if they have similar legislation.
where is the rental? is the market that slow? i'd pass on this one.
because as soon as some issue comes up that 'costs money' their friendship might be over and they'll be behind on rent and you'll be back there in 6 months cleaning up the mess and out a few grand.
renting to friends is one of your less stable prospects.