The The Cloud-based Server Manager offers a minimalist Server Whitelist Manager and is intended to assist with the basic task of adding and removing players from your server's whitelist. The interface operates using Steam IDs and allows you to quickly either add or remove players from your server whitelist, using the player's Steam ID.
If your whitelist is already established, you can click the button to 'Fetch Current Whitelist', displaying all of the players who are currently on your server's whitelist, as well as an option to remove them from the list.
How Does it Work?
When the Whitelist is empty, your game server will continue to act as normal. However, once atleast one player has been added to the server whitelist, the server shifts to "Whitelist Only". Any player that is not on the whitelist, will not be able to join. In other words, the whitelist become will be enabled so long as there is a player listed within it. Once all players have been removed from the server whitelist, and it is emptied, the whitelist will be deactivated.
Screenshots (click to enlarge)