Are you able to make requests between instances on non-public ports? As someone else pointed out Memcached infrastructure typically won't sit on your local webserver.
So lets say you've got memcached, mysql and a bunch of webservers.
On the memcached security group you open 11211 to the webservers group
On the db security group you open 3306 to webservers
On the webserver group you open 80 and 443 to everyone.