How to add a existing user to existing group using

https://www.cyberciti.biz/faq/howto-linux-add-user-to-group/

Add existing user tony to ftp supplementary/secondary group with the usermod command using the -a option ~ i.e. add the user to the supplemental group(s). Use only with -G option:
# usermod -a -G ftp tony
In this example, change tony user’s primary group to www, enter:
# usermod -g www tony