Ok I found Cass and I accepted her quest. I forgot how this works but if you have Boone as a companion and she says no (to join you) because he is there, does the dialogue option come up again? Should I "put Boone to sleep" first?

Just tell Boone to go back to the Lucky 38 first. You can recruit everyone and just send them all back to the lucky 38; they'll wait in the presidential suite instead of scattering across the map like in FO3.