I remember several years ago that I used red rocket truck stop as one, and my brother used Egret Tours Marina as one too, though now I use the Castle for the companions
Typically I just let them go back to their default locations rather than send them to a settlement so that they don't count towards my population limit. After all, shouldn't Piper be going home to check in on her sister every once in a while? Preston is the only exception, I leave him at the Castle, most appropriate location for him.
They have homes. They don't need to wander rough settlements aimlessly when they already have places to live. The only exception was MacCready who was a regular tagalong.
