Login with a user that has an admin profile to https://login.inventivedesigners.com/

Select Manage - Users

In the top right corner, press the Add user button

Provide a username and password and select Add user

In the users list, select the user you have just created and edit the user

On the Summary tab, provide the user's email address and press the Invite email button

On the Security tab, add the roles you wish to give to the user or select a group to add the user to

Save the changes on the Summary tab