Can you use followers at any time? Example: I have St. Josephine to replenish my lives. The Sorceress lands on me and says she is taking her. Can I use her to get my lives or do I have to give her up? I know we have a time line for using spells, but I'm not sure about abilities or followers.

I would say that you lose St. Josephine the moment the Sorceress declares her intention to take her. That's the risk you take by holding onto her whilst there is a character about who can steal her from you...

I agree with Talismanisland. If the Sorceress nabs St. Josphine then you are too late to use her.