how to add a user into a group in linux? code example

Example 1: linux add user to group

# Change the primary group a user is assigned to, note the -g here.
# When you use a lowercase g, you assign a primary group. When you 
# use an uppercase -G you assign a new secondary group.
usermod -g groupname username

Example 2: linux add user to group

# add user to additional group

usermod -G groupname username