First need to login to the Management portal. On first signup you will receive an email with login details to your panel.
Next, you will want to open the File Manager
Navigate to the "Saved" folder
Right click the file and select "Edit"
adminlist.txt - Put the ID of player to make them an admin
bannedlist.txt - Put the ID of player to ban them from the server
permittedlist.txt - Put the ID of player to whitelist them
You can find a players by pressing "F2" in game. You will see the players ID listed next to their name.