The only way to find the connected workstations is reviewing the MOM log. You need to do a grep/find for the following string:
[Manager.PostOfficeHub] Connected To: Node=Workstation
in the MOM log. This is a [VERBOSE] message.
Use the following setting in the IntroscopeEnterpriseManager.properties to log this message:
There is no built in command to find the hostnames of all connected workstations.
In the Investigator you can see the number of workstations connected under
SuperDomain | Custom Metric Host | Custom Metric Process | Custom Metric Agent | Enterprise Manager | Connections: Number of Workstations