The group and the clan are two separate functions. To be a part of the group you just have to join it. You join the clan after you join the group. If you are in a clan, it's name is displayed along your player card. You cannot do this on the companion app. Visit the full site for this group instead and click on the "Set as Playstation/Xbox Clan" tab on top to become a part of this clan. The "Playstation Clan/Xbox" tab let's us check our clan roster.