https://www.gamespot.com/articles/pokemon-gos-invite-only-ex-raid-type-begins-testin/1100-6453026/

Along with the news that three new Legendary Pokemon are on the way to Pokemon Go, developer Niantic has announced that it will begin field-testing the game's new Exclusive Raids very soon.

Now dubbed EX Raids, these differ from the standard Raids in a few important ways. Most notably, players will need to receive an invitation in order to participate in one. To get an invite, you will have to had completed a Raid at that location "recently." Niantic says that players will have "advance warning" before an EX Raid Battle begins to give them "ample time to coordinate with other trainers."