The purpose of the Shared sidebar is to show you a list of documents that other Coda users on your network are sharing.
To connect to a shared document, double click it. While connected to a shared document, you will be able to see the other users editing the document in real-time, and vice-versa.
If the shared documents you wish to connect to are not located on your local network, but are accessible by a public IP address or hostname, you can connect using the Connect To… button located at the bottom of the Shared sidebar. These documents will not appear automatically in the Shared sidebar.