It’s very easy to add a contact in imMail. To do that, click on the Contacts tab upper left on your screen, then “Add a Contact”. Type the email address of the person you want to invite and then click on “Send”.
If the contact you are trying to add is not an imMail user, the system will pop up a notification asking if you want to send an invite. If you want, click on “Invite” and the system will send an invite by email.