Note: it would be easier for audio/video sync if the OBS virtual camera had audio built-in, or if it had an optional plugin to allow it.

For multiple users on the same computer, you would need to install per each user.  When installing VB-cable on the next user, it destroys the device and recreates a new one.  It is easier to install VB-Cable using the INF file for each user, then going in and configuring the monitor interface.

1. Run the setup_x64.exe
2. Login as each user and run the INF.  For windows 10, install win10.INF.
3. Logoff each user.
4. Log back on as each user and open OBS.
5. Pull up the Settings for audio in OBS.
6. Set the monitor device to "Cable Input (VB-Audio Virtual Cable)" and close the settings window.
7. Select the gear below the mixer board and change each device to "Monitor and Output".  (When you add a new device, you will need to change this setting.  You might need to visit each scene to get the audio settings for every device.)
8. Now have fun!

Note: If you have to add a user and the VB-Cable doesn't work, you may have to revisit each user to reassign the monitor device back to the newly installed device.