Look in your bag. I bet it’s there.
You must have a level 30 retail character. The “invitation” is a quest in the anniversary package you get in the mail.
I read about the hotfix, so i moseyed on down there…about 7 min ago. Got in a group…did it…and did get the mount. 
I killed it 55 times yesterday before reset. I’m not upset. The original design was weird.
But I think they changed it for other reasons. Perhaps they didn’t realize how incredibly popular it was going to be as a group event. For some reason they don’t seem to like that happening. Maybe with everybody farming all their alts on it there was a big dip in Shadowlands participation.