Change User/Group Attributes to a File

It's very simple.

For User:
chown username filename
chown username directoryname

Recursively:
chown -R username directoryname

For Group:
chgrp groupname filename
chgrp groupname directoryname

Recursively:
chgrp -R groupname directoryname

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Change groupname and username at the same time.
chown username:groupname filename
Change username and groupname at the same time recursively.
chown -R username:groupname directoryname

Source: http://www.mysysad.com/2006/09/change-group-attributes-to-file.html