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.
If you there are a lot of people you'd like to join your group you could alternatively select "Allow users to join this group" but leave the checkbox for "Make my group visible to all members" unticked, making the group private. A private group doesn't show up in group search results or on the public groups list, but any user with the web address will still be able to join without requiring an invite from you.
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.