how to watch login users from some type of UI

You can use service discovery in admin ad-hoc scripts to retrieve (among other things) list of online users. We recommend using Psi client in which you connect to an XMPP admin account (specified in the init.properties files) and then for that account access service discovery. Once you get the list you list all available commands for the given component (in your case it would be Session Manager) by double clicking that item and then select ad-hoc command that interests you.

Alternatively you can monitor raw numbers of connection through a JMX monitoring.