There are a couple of options to add a user to group(s) in Linux. The commands are gpasswd, usermod and adduser.
I recommend to use gpasswd to add a user to group, which is no harm.
gpasswd -a <USERNAME> <GROUPNAME>
You must use usermod command as follows:
usermod -aG <GROUPNAME> <USERNAME>
If you forgoet '-a' option (only with '-G' option like usermod -G ...), note that surprisingly it overwrites the existing group information.
If you are on Ubuntu, you can use adduser
adduser <USERNAME> <GROUPNAME>