Addendum to add Room mate?

[Kelsey Herbst]

Hi!

My tenant would like her adult son to move into her spare room temporarily while he looks for work and an apartment of his own. 

I figured adding an addendum to the lease to add him as a tenant with joint and severable liability would be sufficient, but I have no experience with writing any addendums yet. 

Would anyone be willing to share any template/resources for verbiage in adding room mate addendums to current leases? 
Any other suggestions for a short term tenant addition like this?

Thanks in advance!