I fill 2-3 spots at the beginning to guarantee my rewards and only within the last hour (if I'm around), try to add more if there are empty spaces. I realize everyone can't get on right when it opens though and even if everyone could, there'd be people that still fill entire zones by themself. I don't really know of a solution to that situation though, but it sounds like it worked out in the long run for you.
For sure there's been times guilds I've been in had around 18 out of 35 signups get 0 even when there weren't even full zones. I've even encouraged in chat to drop a couple teams down to get rewards, but it doesn't help. It's really bizarre to me.
There's a real and interesting theory that some people are actually NPCs in life and have no consciousness. Maybe some of the ones I see in guild aren't programmed properly.