Beginning on September 6th, Niantic will start testing the next set of Raid Battles within Pokemon GO. These Raid Battles are being called EX Raid Battles (Exclusive Raid Battles), and they are unique compared to the Raid Battle presently found in Pokemon GO. EX Raid Battles will be by invite only and done so in advance of the Raid Boss appearing, which is said to allow enough players to group together to defeat said EX Raid Boss. In order to receive an invite to a EX Raid Battle, a player needs to have defeated a regular Raid Battle at which the EX Raid Battle is taking place. At the present time, it appears that only the likes of certain Legendary Pokemon (such as Mewtwo & Ho-Oh) will be done as EX Raid Battles.
During this period of testing this new type of Raid Battle, only certain locations in the world will have access to them. Even though most fans are disappointed by this, Niantic, the developers at Pokemon GO, are looking to work out any and all bugs before releasing EX Raid Battles worldwide. At this point, there has been no word on how long this testing period will last.