You must be registered to be able to invite friends. Go to the “Find Friends” page. To find this page, click either on the “Friends” button in the menu in the bottom-left of the lobby screen or on the “Invite Friends” button in the Timed Bonus.
On the “Find” page, you can invite friends via Facebook. This requires the app to be linked to Facebook. Alternatively, you can send your friends an invitation link via e-mail, text message or another messaging app that is installed on your device.