Getting onto the PUBG Test Server is actually very simple, but only available for players on PC (this will be more relavent when the Xbox One version launches on December 12th).
To play on the test server, you will need to have purchased the Steam Early Access version of Playerunknown’s Battleground. You cannot access it if you’re using a shared account’s game library.
Open your Steam library and under your ‘Games’ tab, locate “PLAYERUNKNOWN’s Battlegrounds (Test Server) and install it.
Once the installation is complete, simply launch this version of the game – you’ll need to create a new nickname and design a new character before jumping into a match.