How do i join to Global risk?

http://crossfire.z8games.com/events_gc_choose_side.aspx

Choose your Faction, you'll be taken to the Clan page of the Faction you choose. On that Clan page you'll see the "Apply" button. Click that, you'll be asked one more time if you're sure you want to join. Once you join a Faction, you cannot change your mind and join the other. Joining a Rival Faction does not affect your membership in your normal clan. Remember, the Ribbon for joining a Faction may not appear on your Profile for a few days.

You can only join a Rival Faction in the few days between Monthly Missions. The last mission ended May 31, the next mission should start around Jun 9.