I found Cass (an npc companion) but I don't know how to recruit her - went through all the dialogue options... anyone know what I have to do to get her to join?

i have the same problem

I never ran into Cass, but where is her location?

Mojave Outpost.

To the OP, it may have to do with your Karma. I'm at "Very Good" and all the companion NPCs are like "No".

Early parts of FO3 I had better luck at "Neutral", it maybe the same situation here.

Later you get her to quit her trade route through a crimson caravan mission. After I convinced her to give up her work, there was an option to take her along.

Shes a sixeh companion, and she likes to get drunk (bonus there)