To delete an account group on your Mac, please do the following:
'Manage' (Or select the drop-down from your Outbank ID: Edit Groups) > 'Edit Groups'> besides group name > 'Delete Group'
You can also open the Edit-Group-Mode using the shortcut "cmd + Shift + n".
A group gets also deleted when you move the last account from this group to another group.
An 'account group management' like on iOS is currently not available on the Mac.