Option 1:
Use SmartLog and select only logs which has action "Log in". You will get all users connected to the gateway. You can search also specific time range (24 hours).
In addition, save IP addresses of connected users and search those IPs to see what they accessed.
SmartLog has very nice overview of resource usage.
Option 2:
Use "fw log" on log server or management. I dont know exact command from hand, but I can give it to you once I am at work.
Option 3:
Use connection table with relevant info about connected users. This can be done only from gateways. This method is "real-time", so you can search only for users which are currently connected to the gateway.
Kind regards,
Jozko Mrkvicka