Agregar usuario al grupo Linux
sudo usermod -aG <groups> <username>
Clever Capybara
sudo usermod -aG <groups> <username>
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.
groups
groups userName-Here
groups $username
groups userName-Here