How do I change a directory's group (or owner) on a *nix machine?

chgrp usergroup somedir

If you want to change the directory's contents too, simply append the -R option to the chgrp command as shown below:

chgrp -R usergroup somedir

Note that somedir could also be a file (it doesn't have to be a directory). Also, if you want to change a directory's owner simply switch chgrp to chown. The options between the two commands are the same.