Unapproved tenants have moved in, what to do now?

[Gloria Mirza]

I have a lease with 4 tenants to rent a house.  Gradually 3 of the 4 tenants moved out and the one remaining tenant has signed on 3 new people to move in without my authorization.

My tenant has paid on time and not been much of a hassle so I would like to give him a chance.  What should my next step be?  I think the best thing to do would be to run credit reports on the 3 new tenants and add them to the lease if they pass.  If they fail, I need to evice all of the tenants including the original one.  What are your thoughts?