Set up and manage access requests
I found a very detailed article
on how to configure the access requests in SharePoint 2013.
http://office.microsoft.com/en-in/sharepoint-help/set-up-and-manage-access-requests-HA103456596.aspx
http://blog.cloudshare.com/2012/12/09/how-to-configure-the-site-access-request-in-sharepoint-2013/
I hope the above information will help you to resolve the issue, in case of any queries/questions regarding the above mentioned information then please let me know. I would be more than happy to help you as well as resolves your issues, Thank you.