In ark you can set what players you allow and don't allow on your server. Ark calls this their Exclusive Allow List.
First we'll want to head to /ShooterGame/Binaries/Linux here we can make a file called PlayersJoinNoCheckList.txt. Here we can enter the steam IDs for everyone we would like to join the server
You can find a guide on how to find your steam ID here: How to find your Steam64 ID
Now we have all the players listed in the file we need to enable the check. To do this head to the server Overview page and then to the Additional Arguments box. Here put -exclusivejoin .
Now reboot your server, and only the players listed in our file will be able to join the server.