You should create custom scopes and add conditions on those scopes
limiting results to those coming from the specific URLs. When
searching each scope, the users will only see results with URLs
containing the starting URL you defined. You can also limit the scopes
the users see in the scope selector (or hide the selector), you need
to replace the search box embedded in the master page (it's a delegate
control) with your own search box (same control, just with your own
parameters).
Just go to search settings (under SSP) there you will see a section
for scopes. create a new scope and add a condition requiring the URL
to contain your site URL. compile the scopes and add them to the site
collection search box (under site collection settings --> scopes)
http://stackoverflow.com/questions/3287183/limiting-sharepoint-search-engine-to-a-specific-subsite
however if in future they want to use People Picker feature it only scoped for site collection as far as I know.
hope it helped :)