I'm pretty sure you need to go to Bungie's website and sign in with your PSN information. From there, click on the Destiny icon at top left of screen. Click on groups. Select the group you want, this will take you to the group's page. Underneath the group's name and motto on their page there should be 3 options. You should select the one that says "Set as playstation clan". If they don't already have max number of people allowed using their clan-tag you should be golden. You'll get an onscreen message if you cannot use said clan-tag.