Member Alias Addresses
You can only access Member alias addresses if you have a member account and you are logged in. See Quick Start: For Members
Quite often people have several email addresses and even though they only want to be a member of a group once (so they only receive group messages once) they do want to be able to send to the group from any of their addresses. This is where member alias addresses come in.
A member can add as many alias addresses as they like, they will be able to send messages to the group from these addresses but they will only receive one copy of messages sent to the group.