I wanted to dismiss cait to my sanctuary settlement but it won't let me select it, it's greyer out. I have plenty of beds and she goes back to combat zone.. Why?

Do you have too many people? I believe there's a cap on the number of settlers you can have in one settlement, something like 10+(charisma)

^ what pup said.

A good tip I've seen is to have your Companions wait at one of the smaller Settlements - that way they're all together and easy to find when you want to swap out.

Thanks I'll look into that! I have 17 right now