How much to raise rent for additional tenant

[Mike Palmer]

Hi all:

I have a unit rented to some college students (in Ogden, Utah). So far they have been good tenants. One of them is a friend of the others as was planning to move out of town and was initially only a temporary tenant, so I gave them what I feel like is a really good deal because she was only staying 3 months. Well now she has decided she would like to be added to the lease and stay the remainder of the lease. 

Has anyone dealt with this situation? What is typically a fair amount to charge for an extra person? It was rented as a unit (not per room), but I also pay utilities and build that into the rent. What would you do?