sudo usermod –g <group_name> <user_name> Note that the lowercase -g flag is used for primary groups and the uppercase -G for secondary groups.

-a: Adds the user to the group without removing the user from other groups.

-G: Specifies that the following argument is the group to add the user to (multiple groups can be specified separated by commas).

Replace groupname with the name of the existing group and username with the name of the user you want to add to that group.