Complete the Van Graff Quest
Cass will not join you until you finish the Van Graff questline. Travel to the NCR territory and complete the related objectives to unlock her as a follower.
Travel to Freeside with Cass
Once she is a follower, fast travel or walk to the Freeside entrance. Cass will move with you and remain in the area as long as you stay together.
Manage Her Behavior in Freeside
Keep her close during combat and avoid hostile actions that might cause her to leave. You can dismiss her safely if you need to explore alone.
For more details on follower mechanics, see the official Fallout New Vegas wiki at https://fallout.fandom.com/wiki/Fallout_New_Vegas_Wiki.
