I got an EX raid pass yesterday for 4.45 on Christmas Day. The gym it was issued for was one I've only raided at two or three times, and one of those was my first Groudon which I did the day before. We all joked at the time that it was "a Mewtwo gym" because there have previously been two EX raids there, but I don't think any of us expected it to result in an invite.
My son also received a pass, but my sister, who always raids with us (and does a lot more besides) didn't get one, despite the fact we were all in the same raid group. That's the third raid she's been at where most of the other players got EX passes and she didn't. She's L38 with double the amount of legendary raids that I have, and many more gold gyms (including at the gym where the other two raids were held where she missed out on a pass). Needless to say, she's livid. I hate that our little family raiding group has been split up like this. My son can't feel happy because his Auntie didn't get a pass.
We are seeing a pattern now, though. Just three of our local gyms seem to be cycling in rotation, and invites seem to be split between those who did a big group and people who did solo raids on different days. So we think my sister needs to raid those gyms several times a week, regardless of what the raid is, to try and get a pass. It certainly helps to cut down the aimless raiding we've all been doing in big groups all over the North Shore in an attempt to "trigger" an EX raid there. Although, I guess if we only hit those three gyms in large numbers, it will become a self-fulfilling prophecy.