How to add an existing user to an existing group?

How to add an existing user to an existing group in Linux?

We can use usermod to change/add user group for users.

# usermod -a -G group_name user_name

-a, –append

Add the user to the supplementary group(s). Use only with the -G option.

-G, –groups GROUP1[,GROUP2,…[,GROUPN]]]

A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace. The groups are subject to the same restrictions as the group given with the -g option.
If the user is currently a member of a group which is not listed, the user will be removed from the group. This behaviour can be changed via the -a option, which appends the user to the current supplementary group list.