There's a bug with companion pathing at Hoover Dam. If you go out of the visitors center using the ground floor entrance your companions will attempt to run upstairs to the observation deck exit. Sometimes they'll make it if you run around to that door, sometimes they get "stuck" in the visitors center. Until this is fixed I suggest using the observation deck exit.
Not the stairs which require a key, but the door next next to the "hard" level locked double doors upstairs in the visitors center.