How to Whitelist Dead Matter Server

In order to enable whitelisting, add the following section in your deadmatter/Saved/Config/WindowsServer/Game.ini file from FTP (or just press the Game.ini button under Advanced Config section)

[/Script/DeadMatter.SurvivalBaseGamemode]
WhitelistActive=false
+Whitelist=MyGUIDHere
+Whitelist=AnotherGUIDHere

Change the MyGUIDHere and AnotherGUIDHere values to match your whitelisted GUIDs from Steam. Feel free to add more in the same format.