Adding additional (new) tenants to a lease, implications..

[Garen T.]

I've had tenants in a SFR for almost a year. It is 3 adults and they all are signers on the docs and had credit history pulled on each one of them. It is a father, his adult daughter, and a family friend. They have paid on time, or prior to the date and appear to be decent tenants. Nothing great, but nothing done so far that I would not renew the lease. I have not visited the place in about 5 months, so I am due for a walk-thru.

They approached me this week wanting to renew the lease and add an adult to the documentation. It sounds like the daughter's boyfriend has moved in, and the dad wants to put him on the responsibility for the lease to make it more official, which seems very reasonable. 

All three of the existing tenants had good credit and nothing that would keep them out. 

My questions for help:

1. Should have have the new tenant sign on the lease, and reasons why/why not. 

2. Should I have a credit check run against the new tenant? Why/Why not. 

3. What if the credit check for him comes back bad? If he is going to be responsible for 1/4 of the payment, what does really getting his credit run add to this deal for me? 

I do like getting him onto the lease since if anything goes wrong, it gives me one more person to go after, but I'm wondering what other advantages there are.

Thanks