I've added a new function to enable current registered members of the site to automatically invite friends and colleagues to join the site. The new menu option available once logged in is (helpfully) called:  "Invite your friends and colleagues" and takes you to a page where you can simply enter the email address of the person you'd like to invite to the site, together with a personal message. Upon receipt, all they need to do is click a link to be taken to the 'Register' page.