I'm beginning to think that there's some glitch in the game that crashes people, or otherwise keeps the group from being formed...there's no way that someone is actually declining the invite 20 times in a row, right?
Yes, there is. It's no glitch.

It's because of people that want an in-progress run rather than one starting from the beginning. If you hit accept when the queue is ready and you see the whole 0/6 0/6 0/12 thing come up showing that it's waiting for all 24 people to accept, that means that it's not an in-progress run so they immediately back out.

Is it dumb? You bet. Does that stop people? Course not.