Hmm. That should work, and a quick test locally shows mine working correctly.
There are several things to verify.
1) Examine the actual userfile to see what the "groups" line says.
2) Examine etc/GroupIdTable to verify group 0 exists (ioFTPD by default) and that iND only appears once and isn't group 0
Using your example above does "site chgrp sitebot SiTEOPS" remove the user from the group?