You need to add an existing user to a linux group
You use the usermod comman with the -a and -G switch to do the job.
usermod -a -G thegroup theuser
This will add the existing user <theuser> to the group <thegroup>
About the Author
Wim Peeters is electronics engineer with an additional master in IT and over 30 years of experience, including time spent in support, development, consulting, training and database administration. Wim has worked with SQL Server since version 6.5. He has developed in C/C++, Java and C# on Windows and Linux. He writes knowledge base articles to solve IT problems and publishes them on the Lubby Knowledge Platform.