TX - Adding roommates to a tenant's lease

[Anthony Varner]

I'll get right to it...

Guy moves in to my rental.  All is fine .  Three months later I've discovered he has moved three roommates into the place.  I have no idea who these people are, where they work, credit, background, etc.  I spoke with a real estate agent friend of mine and he said I need to get everyone on the lease asap to protect myself in case I need to evict.  He said you can't evict someone who isn't on the lease.  

Now, the question is, should they all have to do a rental application and pass both background and credit check before being allowed to stay?    

Or could they legally be added to a lease amendment listing them as "occupants" then whatever happens - the original tenant is held liable?

Or should a new lease be drawn up and have all four people listed as "tenants" and all jointly and severally liable?

Thanks so much.  I really hope somebody out there has some wisdom on the subject and is willing to share it with a newbie.

-Anthony