There is a current issue with the Rival Factions Ribbon.
If you attempt to join a Rival Faction side, it will likely automatically put you in GR. And then it may not give you the ribbon.
We are fully aware of these issues at the moment and are working to restore both new Rival Factions Missions, Stories and the ribbon problem, and of course the ability to join a side!
Please be patient, we will have them working soon. Thanks!