9.13 Ship Permissions

Space travel and technology is so sophisticated that any size ship can be safely captained by a single person. Crews are no longer necessary, in the sense that multiple roles do not need to be filled separately (for piloting, navigation, lookout, and so on). 

Multiple people can certainly board a ship, but serve as spectators only.
(Tip: You can turn off ship spam as a passenger using: 
CONFIG IGNOREUNOWNEDSHIPMESSAGES <ON/OFF>)

The easiest ways to allow someone else to board your ship are:

ALLY <person>      Allows them access to any of your ships, as well as other 
                   things enabled by adding them to your ally list.
FOLLOW <person>    Get them to follow you and they will board any ship that you 
                   board.

You can, however, get even more fancy with your boarding permissions by setting up PERMISSIONS. Using this system you can specify things like faction, level and class restrictions. Once you have created a permission, you can apply it to a ship you are on using: SHIP BOARDPERMS <Perm#>. Once you've set one up, they will show as a list if you just type SHIP BOARDPERMS.