If your club want to ensure that no player can create an account without a parent connected, you can change the setting for this as an organization admin.
Navigate to Settings while in the top level group, and find the setting called Allow player signup. This setting is ON by default.
When disabling this setting, the option to sign up as a stand alone player in the account creation process is hidden - ensuring that any players or parents creating accounts from either group codes or registrations are selecting the correct role (parent creating a player account).
How do I add a player with this setting disabled?
If you still have the need to add a player directly (e.g for an adult team), you can simply use the email invite method of creating a direct invite to the user - which will create them as a player in the group.