SQL Concurrent Users Rules
These Rules monitor the number of users connected to the SQL Server machine at any one time.
SQL-DMO library must be installed on the monitoring engine. It is generally installed as part of SQL Server client package, but do check sqldmo.dll is in drive:\Program Files\Microsoft SQL Server\80\Tools\Binn
You can use this Rule to monitor the number of connected users to either the whole of the SQL Server on the machine, or to an individual database.