Once Source Insight is installed on a machine, it is available to any user on that machine, but the licensing differs depending on whether the operation system is a server or not.
On desktop operating systems, Source Insight is licensed per-machine. There is a license stored for the whole machine. Once installed and activated, you can run Source Insight from any user account on the machine.
On server operating systems (including Terminal Servers), Source Insight is licensed per-user. It must be activated separately for each user, and there is a license stored for each user.