Can you clarify this for me. Are you talking about sites
in the Windows 2000 sense, as in different subnets are
assigned to a site? If so you can't specify a user as
belonging to a site, as the site they belong to depends
on the IP config of the machine they are using and the
sites defined in AD Sites & Services.
I think what you may be meaning is you have sites in
the "a site at London, a site at Liverpool" sense, and
maybe you have an OU to represent each one of
these "sites" (offices)? If so, just right-click the user
in AD Users & Computers and select Move and you can
select where to move them to.
If none of the above, please explain further.