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
More Instructions on Nix Craft