I am remaking one of my Fallout 3 mods I made along time ago for New Vegas. One thing I never got down was how to get a companion to fast travel with you are follow when or enter a new area.
If your folllower is on a "follow" type package, most of this will tend to work without you needing to do anything else.
But, there can be problems with followers being able to follow the player through load doors sometimes. It can be a pain to deal with. One way to deal with it is by scripting the doors, In Fallout NV, I think that Obsidian themselves is already scripting at least some of the doors. I saw a generalized sort of script which looked like it could have been placed on the various problematic doors and it would have worked well enough. In principle, you would want to tap into that script and add your follower's REF to it in the appropriate way, but also, understand that only one mod can edit the door script at a time. So if you tried to use another mod which also wanted to use those scripts for ITS follower, that person's script won't be moving your follower through the door. You'd want to be aware of that.