Not all social networks are setup to be locked to new users. If your network is locked then the only way new users can be added to the network is by invite only. You do this be entering an email address this will be then sent to the recipient with a special link that allows the recipient to sign up to the network.

Email
Enter the email address of the friend you wish to invite to sign up to the social network.

A list of emails is displayed of friends and their status you have invited.

Email - The email address of your friend
Email Sent - The date the email sent
Status - A tick  signifies that the link has been received and sign up is in progress. A cross signifies that the email has been sent but the link has not been clicked.

Delete - Remove the request.