Niantic has finally clarified the requirements that need to be fulfilled in order to receive an EX Raid Invite. Unlike the randomly distributed test invites, the new requirements set is clear and simple.
TLDR: you need a high-level Gym badge and a large number of completed Raid Battles in order to be more likely to receive an EX Raid Invite.
EX Raid invite requirements
The following aspects influence the likeliness of receiving an EX Raid invite. Pay special attention to the Champion Badge — you want to reach Silver Ranking as soon as possible.
|Gym Badge||Trainers with a high-level Gym Badge are more likely to be invited to EX Raid Battles taking place at that Gym.|
|Champion Badge||Trainers who have completed a larger number of Raid Battles are more likely to be invited to EX Raid Battles.|
In November 2017, Niantic published a Developer Insights blog post, offering an honest explanation of this system and why those two factors were chosen as the basis of likeliness to receive an EX Invite:
When the feature first entered field-testing, the system randomly selected a set of Trainers who were eligible for the EX Raid Battle. By taking Gym Badge level and total Raid Battles completed into account, we’re able to give Trainers who actively engage with the Gym where the EX Raid Battle is taking place and those who have invested more time into the Raid Battle feature a better chance of being invited to participate.