I initially sent a notification email to a person about a shared folder, but now I would like to make this person a group member, but it is not letting me. I keep on getting the error message: Creat sub user failed, the email is already used! Please help. Frustrated.

Hi, this is normal as the person is already a DriveHQ regular member. In this case, he must agree to become your group member. You can use the "Add existing user" link in the Group Admin to ask for his permission to become your sub-user.

You can also create a sub-account with a different email address. For group sub-accounts, email confirmation is not enforced. So you can use any email address.