What are the rules by which EX Raid passes are given out in Pokemon Go? Is there some way to maximize my odds of being able to get one?
I got one last Community Day, and while I caught Mewtwo that time, I wanted to figure out how I could get another so I could get some more candy and/or a shiny version.
Best Answer
Your best bet on getting another one is to coordinate with a local group. The Silph Road subreddit has put together a map that will help you find the closest discord group to your area. These servers will typically have an EX Raid channel, as well as other channels, such as one for ditto spotting. I only learned about this map in the last two days, although I've been on two different discord groups in as many months (one in CA and one in PA).
However, we also know some of the rules by which Niantic uses to give out EX Raid passes, but there's still some guess work. Niantic themselves have said that battling and raiding at "sponsored gyms and gyms in parks" as well as having a high raid frequency and high gym badge ranking will increase your chances of getting an invite. How much this affects your odds, we still don't know.
What we do know:
Even after all of this, not all trainers that participate in a raid at an EX-eligible gym will receive a EX Raid pass. The statistics work being done on the influence of gym badges and so forth doesn't yet have enough data, and that data collection may be flawed (as people who do not receive an invite are less likely to report).
Different strategies are around, but the one that makes sense to me is to just raid every gym within a given "super cell" at least once. However, coordinating with your local group will be important. They will likely already know which gyms can be EX eligible and have a strategy for raiding successfully.
One interesting side note: if you ever get a shiny raid boss, the catch rate is 100%. Toss a pinap at it and throw clean; easy 6 candies.
Other sources:
Pokemon Go Press
Pokemon Go Hub
Pokemon Go Hub (more comprehensive)