If you logon as a Group Admin account, you can see a Group Administration link. Click it, it goes to the Group Administration main page. From there, you can invite an existing DriveHQ user to your group. Please note you cannot add a DriveHQ users directly. The user must explicitly accept your invitation to join your group. By joining your group, you gain full control over his account as a Group Admin account.
In the future, if the user should be removed from the group, you can either disable the account, or remove the account from your group, or remove the account permanently.