Number of active users on 3825

I guess you could dump the NAT active dynamic transation info using the Cisco IETF NAT MIB. The only issue with his is that you can still have a translation after traffic has ceased as eachprotocol (TCP, UDP, etc) has a timeout before translation entry is removed from translation table.

You might want to look into implementing NetFlow accounting which would provide traffic by source and destination IP address.

Turn up netflow as merv suggested, with ip route-cache flow on the vlan interfaces. Then do a show ip cache flow which will show you all active sessions. The only downfall is this will be all sessions, so you will see multiple sessions per box. This is probably the only thing you can do to monitor sessions at a network hardware layer.