Log into your OnDOMAIN account.
In the top right-hand corner click on your user profile.
At the top of the page, you should see a section called OnDOMAIN Users, you can invite users to join OnDOMAIN by entering their email addresses.
The new users will get an email inviting them to join your account with further instructions.
If you want to revoke a user's access to your OnDOMAIN account you can also do this by deleting their user name from the Account Users list.