Add User to a Group Command for Ubuntu / Linux

Quick Example Command:
usermod -a -G group_name user_name

Full Instructions

How to add an existing user to existing group via ssh terminal on Ubuntu Linux.

To add an existing user mike to www-data supplementary/secondary group with usermod command using -a option ~ i.e. add the user to the supplemental group(s). Use only with -G option:

usermod -a -G www-data mike
id mike

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