Tenant asked to add new spouse to lease. Implications?

[Eithan Shavit]

Hi BiggerPockets, I'm excited to ask my first question:

I have a 3br condo I'm renting out to 3 tenants with a single lease. It's only been about 5 months in the 1 year lease, and one of the tenants got married and asked to add his wife to the existing lease. I'm pretty new to this and not sure what is the right way to go about adding her and whether it creates any implications down the road. 

Do I ask them to renew the lease? Do I just add an addendum? Would love some advice.

Thanks!