Managing a whitelist allows you to give access to the server only to individual players. It's pretty simple to use; you need to declare every Steam64 you want to give access into a .txt file!
Get the Steam64 of a player
Le Steam64 ID allows the server to recognize a player configured as admin or moderator.
The simplest way to get it is thanks to the Steam Community website. Indeed, each player as a profile page which address is formatted like :
http://steamcommunity.com/profiles/X
The X part in the address corresponds to the Steam64 of the player, which is precisely the ID you need to get to put it as admin on your ARK server.
Adding a player to the whitelist
Each player you want to give access to the server must be declared by his Steam64.
Create the following file on your FTP if it doesn't exist :
ark/ShooterGame/Binaries/Linux/PlayersExclusiveJoinList.txt
Inside you must declare the Steam64 list, 1 line by Steam64, and of course by player.
The modification will be applied at the next reboot. In case the server writes the file when it stops, you can do this modification after having stopped the server.
Enable whitelist system
The activation of the whitelist is about adding a command line parameter to the server.
You can do so by adding a line or create the following file on your FTP if it doesn't exist :
ark/ShooterGame/Saved/Config/LinuxServer/CmdLine.ini
The line to add:
exclusivejoin
Then restart your server, and it's whitelisted ;-)