To make it easier for those who want to sign up and do raids, using the rapid raid functionality (as seen up top), you first need to create a character (the ones that appear in the clan roster). We're testing it out now to see how it works.
In the mean time, to make one, head over to the profile icon (the human figure next to the globe), and click 'view profile'. In there, head into the tab labelled 'characters' (also near the top). Then down to the left side you'll see 'add character'. Click that, and it'll show a list of games. Of course you'll want to scroll down to Destiny.
Further options will appear in regards to details of your character, select the correct ones. Then click "add character". Now you'll be able to sign up through the Rapid Raids system, when raids become available.