Hi Coiote,
When you invite your friends an email will be sent to the account they used to register on WGT. When they receive that email they will need to log in to WGT and click the confirmation link that comes in the email.
If they are not receiving the emails it may be going to a spam or junk folder, so have them check there. They may also have an incorrect email address listed on their account information. They can change their email by clicking the ACCOUNT button and then clicking the CONTACT INFO tab near the top of the page. Just fill in the correct email and be sure to save the changes.
For now I would recommend deleting any pending invitations you have and asking your friends to delete any invitations they have in their email. Then you can re-send the invitation and have your friends click the link in the email to join your club.