Each user is able to 'add' the users they wish to chat with in the DIRECT MESSAGES section:

A user in this list will show a solid colored circle (like Administrator) if they currently have Odoo open in a browser, and a hollow circle (like James) if they do not.

Every time a user is 'added' in this way, an entry is created in the mail_channel model, so you could conceivably pre-populate this model with entries so that all users are already shown in the list for every user (so they don't have to add them one by one) and in this way, use this list and the circles to indicate who is online and who isn't.