Please note that the SubAccounts feature is in beta testing. If you do not see 'Manage SubAccounts' in your Control Panel, you are not on a SubAccounts beta test, don't worry at all. You can completely disregard this help page, and your OPP Site is functioning completely correctly.
If you are on an OPP SubAccounts beta test, the account holder will be able to manage the subaccounts from their Control Panel.