Mewtwo was unlocked globally after being caught in a Japanese stadium event for the game. Thousands of trainers in Yokohama took down Mewtwo just as it was in the first big Pokemon GO Promo video over a year ago! To participate in a exclusive raid battle for MewTwo, you will need to do the following. The difference with the exclusive raids is that not everyone will get a chance to participate, as it's for the more dedicated players. The invite will come long before the scheduled raid, and offer players enough time to coordinate and plan for the attack.
If you meet the criteria for the exclusive raid battle, you will receive an invitation in advance for the raid battle so that you can take it down with preparations. MewTwo is actually the first Pokemon that is perhaps the most high profile Pokemon available in Pokemon GO. According to screen captures, Mewtwo won't make for an easy Raid Boss; the CP is at a whopping 49, 430.
Pokemon GoMewtwo is the first Pokemon that Trainers will be able to encounter in Exclusive Raid Battles.
As for the Legendary Birds, Niantic confirmed that Articuno, Moltres, Zapdos, and Lugia will all be appearing in Legendary Raids around the world for the rest of August. Niantic hasn't clarified (yet) whether they mean new powerful Pokemon that aren't in the game yet, or if they're "new" in the sense that they've not been in Raid Eggs before.