To those who done mewtwo raids:

Everybody was there 15 minutes before hatching.

Then the egg hatched and all the children were like "wow a Mewtwo" and started screaming and running around.

We were about 40, some people suggested to do one groups per team (to maximize rewards and balls). But some other people were afraid not to be enough and to do it altogether and too bad for the few left over 20. Most of the children were running like chicken without a head making it difficult to count. Also many people came without a pass which made difficult the counting.
15 minutes after hatching, there was finally a decision: two groups of about 12, the blue team on one side, and the yellow + red team on the other.
There was a little more confusion because some players wanted to join the "wrong" group because they wanted to be with a friend.
Then started the argument: which group should go private. But one group started and the other took more than 2 minutes to decide the code so no private group was needed in the end.
Lot of screaming when Mewtwo was easily beaten by team 1.
Lot of screaming when Mewtwo was easily beaten by team 2.
Lot of screaming when Mewtwo was caught. At least one children cried for not catching it.
I was surpised that nobody arrived during the 30 minutes between the hatching and the time everybody left.

It's not all that tough, so as long as there are more than 8-10 people there it should be a cake walk (there was a legion at mine, but it was the second one at a sponsored gym). Basically everyone showed up 5-15 minutes early. If you have decent Ttars whether or not the game suggests them should indicate if it has Focus Blast or not. Yes, people tried to split into similar teams for raid groups, but the people who organize on discord or facebook were being jerks about who they would group up with. My group was mostly my team, but was cobbled together with mostly casual parents and their children, even with less than 15 we annihilated Mewtwo before my 4th mon lost a quarter of its health. I would not suggest showing up too close to the end of the raid time, just to be safe. And I would expect the most popular team to be squatting on the gym constantly feeding golden razzes (freaking mystics had no taste for competition).

Me and my little brother (who where the only child shockingly) was att the raid gym 1 hour before hatch to take it over.

2 minutes before hatch a 3rd player arrived.

30 minutes after hatch we had a total of 5 players so we decided to try a Suicune 100 meters away where a 6th with invite showed up.

Walking back to Mewtwo a 7th with 2 phones showed up so with a total of 8 players and 10 minutes left we started our try at the mighty beast.

I've already checked his moves, Confusion Focus Blast.

2 asked what to use.

My level 20 brother used what ever he got from lvl 2-3 raids we have done.

My glass team, 1 Gengar, 2 Tyranitar, 1 Houndoom, 1 Scizor, 1 Pinsir

My second team, 4 Gyarados 2 Dragonite (maybe a stupid lineup order but it worked)

We killed it (legendarys has no gender?) with 63 seconds left, and I was the only one catching it, on the second ball.

I know one player overslept and missed him totally.

Our raid was extremely organized as basically the entire invite list already knew each other from previous raids and Facebook group. One raider set up a Google Docs account where everyone filled in team, arrival time, and level. When egg hatched, we were already broken up into teams and used three private groups. Our Instinct group went first as we had the fewest raiders (7), but we easily defeated him. Total number of participants was approx. 30. And nearly everyone caught him as we all had at least 9 balls.

Don't worry about it. Go with the flow. Are you nervous? ?

Zaarkos pretty much hit the nail on the head with the detailed account of how things go.

I'd just add it's very hard to convince lower level players and/or casual players that a Legendary can be taken down by a small number of players. You've got to understand that if soloing an exeggutor is a challenge, then Mewtwo is like Everest. The idea of having 3-6 level 30+ TTars is completely foreign to them and maximizing damage balls never comes into the equation, and rightly so. Trying to manage the motivations of 40+ people is extremely challenging. It's simply easier to sledgehammer the thing and use the 6% catch rate to your advantage. Think, it's 3x easier to catch than any of the beasts. Even with only 8-9 balls over 50% (closer to 60-70%) caught it and my guess is not everybody had enough golden razz for every throw and I KNOW that a bunch didn't curve (not to mention that the last Ex-raids was before the curve glitch was fixed).

Go have fun, it'll be a circus, and don't over think it.

We had about 60 people. Almost everyone arrived before the egg hatched. One awesome dude took charge and split us up into groups of <20 ppl by team. Then he had each group start at a specified time (about 5 minutes apart). If someone failed to get into their assigned group (app crash, gps issues, etc.), they could join the next assigned group (probably not their same team tho). We had significant gps issues at the gym, so this helped everyone get in a group and beat mewtwo. Everyone was finished after about 30 minutes. A few stragglers showed up after that. I doubt they were able to raid.

My wife and I didn't belong to any organised groups when the raid happened. We arrived at the gym about 20 minutes before the start of the raid and found a few others gathering. By the time it started there was about 25 people around, and a good mix of all three teams. The gym was owned by Mystic. One person took charge and split the raid groups into two. One was all Instinct, one was all Valor. The Mystics were split between the two.

Groups of 12 with a range of levels were easily able to defeat the boss. Around 80% of us caught the boss afterwards, me included.

It was all very well done, nice people, lots of joking and banter, and as a result I've now joined the local Facebook group.

I went to a mewtwo raid organized by my local discord group. We did split into teams. Even though I got twelve balls, though, the bugger still ran.

Take charge and make sure it’s not a shit show. Start by sorting into teams. Elect a person from each team to tally the numbers and levels of their team and determine how many groups from each team will go. Don’t all go at once. I’ve been asking for a show of hands to see who is limited on time. First group will discovery move set. If any stragglers show up, they just fall into whatever group is left. Since our biggest group is Mystic, I’ve been going last with them. We’ve done this at both raids and it’s been smooth sailing. I haven’t read the rest of the responses, but I could imagine it to be a nightmare without some leadership. You are already thinking about it, so I think you are the one for the job.