You have to send the invitations to an email address, or cellphone number and make sure you are including the international area code (choose the flag icon for the country the phone number is registered to).
Your guests will receive an email or text with your invitation. In order to avoid misunderstandings let them know in advance that you are sending them an invitation.
If they do not see the invitation from WEMI in their inbox please ask them to check their spam folder. You can also send all the invitations from your computer. Most of the users are not having any issues when they use their computers.