One way to invite someone to your workspace is to use the share note widget. Using this widget you first share a note and then invite the person with whom you have shared the note. If the invitation is sent along with the link to the shared note, then in the note view, the invitee will see a Sign Up button. On clicking the Join Workspace button, the invitee will be prompted to fill in a registration form. Once the invitee is successfully registered on the workspace, she can post comments on the note shared and create new notes.
The other way to invite is to go to the User Panel and click on the Invite Users link. An invitation to join the workspace is sent by email. Click on the Join Workspace link in the invitation will show the Registration form.