To add an existing user to a secondary group, use the usermod command followed by the -G option and the name of the group:
sudo usermod -a -G groupname seba
For example, to add the user seba to a group named sudo you’ll need to run the following command:
sudo usermod -a -G sudo seba
How to Add an Existing User to Multiple Groups in One Command
sudo usermod -a -G group1,group2 seba
How to Remove a User From a Group
sudo gpasswd -d seba groupname
How to Create a Group
sudo groupadd groupname
How to Delete a Group
sudo groupdel groupname
How to Change a User’s Primary Group
sudo usermod -g groupname seba
How to Create a New User and Assign Groups in One Command
sudo useradd -g users -G wheel,developers seba
Display User Groups
id seba
groups seba