SHELL

add user to group linux

sudo usermod -aG <groups> <username>

see what groups a user is in linux

# this will display what groups the active user is in
groups

linux add user to group

sudo groupadd $GROUP 
sudo usermod -aG $GROUP $USER

linux user groups

 groups $username

What is user and group in linux?

User: the owner of the file (person who created the file).
Group:the group can contain multiple users. Therefore, all users in that group will have the same permissions. It makes things easier than assign permission for every user you want.