Fixture makes it easy to invite users to collaborate on your site.  These users must create a Fixture account and have a valid email address.  To get started, Navigate to settings using the cog icon in the top right of the screen.  Then, click the "Users" tab on the Settings page.

Click "Add User" to add a user.  This action presents you with a dialog to fill in information about the new user.  The email you add must be valid and the user can only register with this email address.  This provides some additional security during the invitation process.  Also note, that the user will not show up your users list until they have registered on the site.