Click on the 'Invite Others' option from the collaboration drop down menu in the toolbar.
You will see a dialog with a session ID for sharing with an iOS user, and a URL for Web sharing. You can copy and share these items or you can click the 'Send Email' button. An email will be automatically generated that you can send to your collaborators. See Joining a Sessionfor more info on how you can join a session on your iOS or Web device. For desktop Web clients, click on the URL in the email to join a session you were invited to.