Is there a way to send her back to Diamond City after she becomes a companion? Don't really want her loitering around Red Rocket, but the only dismiss option are settlements.

Apologize if that has been brought up before. But, there are just to many threads to try to comb through.

I had the same issue. I went into her commands and there was no option to send her back to Diamond City. I then completely backed out of her dialogue and the next thing I know I see the little message in the top left say something like "Piper has returned to (I forget the name of her business)". Might be a glitch, but she's still there doing her thing.

Darn. Oh well, I suppose I had to run into a bug eventually. At least it isn't like some of these other cray cray bugs. I can handle Piper loitering around, if needs be.

Who knows.... I might find a stronghold more to my liking somewhere else.

If you never pick a settlement(tab out) they will default back to where they were

If you do pick a settlement that will be their new default and they will never return to their original spot