What is the contact list and how does it get populated ?

The Contact list is triggered as a result of your network activities: 

Network Tab Counter

This list includes both your followers, the people you follow and your Co-Authors. This list differs from the list displayed when accessing your Messaging Inbox. See here for further details. 

From your contact page, you will be able to send out email invitations and establish contacts with the people in your network.

The contact list is accessible from the compose button  (See here how to a compose message) :