If you select "Users must be invited to join this group" when creating or editing a group, then you have to invite users by e-mail address for them to join via the World Walking website.
To invite people to join your group, login to your account via the website and click on your group. You will see an invite button in the members section'. Just click it and enter the names and email addresses of the people you wish to invite in the spaces provided. The unique link to your group will be automatically inserted when you send your invite; you do not need to know it or enter it.