Use "sudo usermod -a -G example_group example_user" to add a user to a group. To add a user to multiple groups at once, separate groups by commas, like "sudo usermod -a -G group1,group2,group3 example ...

Linux is a multi-user environment, which means more than one user can use the system at one time. Granted, that mostly takes the form of console access (via SSH), because you can't easily have two ...