If you have a mod added (Humanoid) Companion/Follower that will not board the Vertibird:
There are options available to you, without modifying the supported races, to get your companion to board:

First check if the Companion/Follower mod has any additional optional patch files to support Vertibird travel
In that Companion/Follower's Mod Nexus page. Try this first before the next step.

If there isn't one, or they still don't board, then try the following:
Change the  the Holotape setting "Passengers are:" to  "All Followers" (OR MCM setting "Which NPC's will board when player does" to "All Followers") and try boarding the Vertibird again.

Note: FPVertibird considers an NPC to be a "follower" if they are running an AI package which has the flag "Treat as Player Follower" set. It is possible to create a follower without that set, but these will then never board. You could resort to the following "last option" to fix this, However i would recommend talking to the author and getting them to patch their Companion mod instead, as they will have trouble with other mods too...

If this still doesn't work then try the following (but only if there is no other option):
Give the problem Companion a "Passenger Token" (to put them in the passenger seat) or one of the Hats/Rings (if you want them in a particular seat) via that Companion's 'trade' menu. The token/hats/rings are crafted in the Chemestry Station under "Personal Vertibird".
Now change the setting to "Follower OR has a Token/ring" (Holotape or MCM). Now any nearby NPC's with a Passenger Token (or Hat/Ring) will automatically become passengers as well (against their will!).

Note that with this last setting you can give ANY (human/robot/synth) one of the craftable tokens/hats/rings and they will board - even if they are NOT a follower (such as a settler or a Hostile Robot). This method allows you to make any non-companion NPC (say a settler) your Pilot etc.

If a Passenger Boards but then immediately exits, or exits randomly while flying:
When onboard a Vertibird, Passengers run a special (FPV added) AI package that ignores combat and keeps them in their Seat.
However, it is still possible that a mod added Companion has a background quest controlling it that interferes with this so they will randomly exit (especially on takeoff). You will have to get that Mod's Author to fix it.

Issues with Non-human Followers not Boarding:
Even if you resort to the Passenger token option, the Follower may still not board, as Boarding requires animation information in the Passenger's Race record that links a boarding animation with the "isVertibird" destination and seat keyword. Examples of mod Companion that won't board are ones that give a you a 'friendly' Animal Companion Molerat, Deathclaw, Brahmin, Fogcrawler etc (basically any 'Beasts' except Cats and Dogs).

If your Companion's race is not based on one of the existing races that I support, then you will have to create a mod to enhance that Companions race by creating a extension subgraph for that Race with this animation reference.
You can examine the extensions to the Assaultron Race, cat race, etc in the FPV mod to get the idea.

Supported Races:
The following Races should have no problems boarding the Vertibird as they have boarding animations built in (base game, or by this mod).

- HumanRace. This includes Children,  Power armor wearers, and any Race with the "ActorTypeHuman" keyword (like Anime/MLP).
- Ghouls (not Feral)
- SuperMutants
- Synths (all)
- Robots (Mr Handy, Protectron, Assaultron, Eyebot) - their 'race' is defined by the legs they have, not their Torso or Head.
- Dogs
- Cats

The only issue may be if they are a  'follower' (as mentioned above) so you may need to set Passengers are: All Followers.
Companion Eyebots and Cats are generally Followers for example (as the base game never added one of them as a Companion).

Creating your own "seat" allocation items:
This mod supplies Rings/Hats for all the available Seats, but if you want to create your own Seat allocation 'item' then the existing scripts in this mod can be used on any item you create to make an NPC a passenger and sit in a certain seat.

The script fpvertird:FPV_token can be added to any item that can be placed in an NPC's inventory. When the NPC is given an item with this script they will board the Vertibird into the specified seat (defaults to passenger seat if no properties on it). A property on this script called VertibirdSeat defines the seat they take. This (Int32) has values from 0 to 7 and mean: SeatedPassenger,CoPilot,Pilot,RightStanding,LeftStanding,Gunner,Dogmeat,2ndGunner

A second script fpvertibird:FPV_CopilotHat needs to be on an item that is Worn. When the NPC equips an item with this script they will board the Vertibird into the specified seat (defaults to CoPilot seat if no properties on it). A property on this script called VertibirdSeat defines the seat they take - same as the above script.

Note: the FPV_Passenger property is only required to be filled if you want this item to also work like a "passenger token" (i.e. will work on non-companions).

(Updated for V1.6.3-3)