In Site Settings, go to the security tab and  put you email in the EmailAdressesForUserApprovalNotification

You can specify an email address or list of email addresses separate by commas and these addresses will be notified when new users register on the site (very important if approval is required before new users can sign in). Note that in order to be able to approve the users, the recipients of the notification must be in Roles Allowed to Create Users (see the Permissions tab). If you leave this setting blank but require approval for new users to sign in then all members of the Administrators role will receive the notifications.