For SCM 12.1 and above, to find the number of currently connected users you can use a command line utility called hgetusg. Details on this command can be found in the Command Line Reference Guide.
If, instead you need an exact list of which clients are connected to the broker, you can check the list of clients connected to the RTServer. This method will work for all versions of Harvest and SCM. It won't provide your harvest userids logged in, but it will tell you the hostnames of the client machines connected to the rtserver. To do this open a command prompt window on the broker machine and execute this command:
This opens an avenue to send a direct query to the RTServer. To get the list of currently connected users execute these commands:
setopt server_names start_never:tcp:<rtserverhostname> poll client_names run 1 1
Where <rtserverhostname> should be replaced by the hostname of the rtserver.
After you get the list of currently connected clients you can close rtmon with the 'quit' command.