A systems administrator is reconfiguring existing user accounts in a Linux system. Which of the following commands should the administrator use to include "myuser" in the finance group?

`groupadd` is used to create a new group, not to add an existing user to an existing group.

`groupmcd` is not a standard Linux command for managing groups or users.

`useradd` is used to create a new user account, not to modify an existing one, and `-g` would set the primary group for a *new* user.

The `usermod -aG finance myuser` command is the correct way to add an existing user ('myuser') to an existing supplementary group ('finance'). The `-a` option ensures the user is appended to the group without removing them from other existing groups, and `-G` specifies the supplementary group(s) to add.