Add new tenants to the lease?

[Jon Klaus]

I've got a large house (5 BR) with a couple and 2 kids in it. They want their friends (another couple) to move in with them to share costs/housing. The new couple has good income; enough to qualify on their own. I'm open to this and would qualify the new couple like anyone else, but do I add their names to the lease with an addendum? What else should I consider beyond my normal screening?