How those work doesn't sound too different from existing Raid Battles, but you'll need to get an invite. Meaning, if you want to snatch the psychic-type creature for yourself, you'll first have to complete a raid, defeat its boss and then head to the Gym where the Exclusive battle will happen.
"The invitations will include advance warning of when the Exclusive Raid will take place, giving them ample time to coordinate with other Trainers before taking on the powerful Raid Boss," developer Niantic writes.
Our colleagues at Engadget Japan say that once you defeat Mewtwo, capturing him isn't all that difficult, either.
Mewtwo is just the start, as other legendary creatures will start appearing over the next few weeks. Speaking of, Niantic is adding Articuno, Lugia, Moltres and Zapdos to raid battles from August 14th through the end of the month. Happy hunting!