New landlord and possible tenant situation already! Advice please

[Stephen Neto]

I'm a new landlord, haven't even collected first rent from tenants yet on a 3 unit in a city of about 100k population. Neighborhood is rough in an ethnically diverse section, but property price and rents seemed good. Second floor tenant, who lost her lasts months rent because old landlord used it when she was behind, seems to have invited other people to live in the apartment when she found out there was a new landlord. Got this Info from first floor tenant. She already lives with her girlfriend. 

New tenants are not authorized unless they go through me with an application. How should I go about this?

Should I wait to see if they at least pay their rent? I am assuming she needed help with the rent. But I don't like the fact that she brought someone in without asking me.  I am doing property management myself or thought I was going to. But its stuff like this that makes me want to hire a PM co to deal with that type of BS.

Oh and I know they smoke pot in the house because last time I was there, I smelt it.