Alright, get this.
When renown resets, which is bugged in itself already, you can't sign up for minis and if you just started one you are unable to start or leave it, it'll just crash. After this crash you'll have the groupbug. And if you enter a mini (not sure if it has to be the same) you'll have your bugged group on top of the one you really have.
I just had JS with 3 Groups in raid, see link: http://imgur.com/1IovId0
Bugception! Awesome!