Currently you cannot join a Rival Factions Side due to certain issues, but here is how to still do so and receive the "Which Side Are You On?" Ribbon.
You do not need to leave your clan, just follow the instructions provided.
1. Login on the Z8Games website.
2. Click on either the Black List or Global Risk clan application link according to the one you would like to join.
Black List Link
Global Risk Link
3. Hit submit to join the clan.
4. If you applied correctly, you should receive the follow message:
"You have successfully joined the Global Risk/Black List clan.
You are now being redirected to the 'Rival Factions' event page"
5. Check here (My Profile Page) to see if you received the ribbon:
Enjoy your new ribbon.(If you do not already have it)