Sites in 'Find People' category are sites which find People. This is the reason why search sites vary in the 3 different categories. Now that you have search sites, select any number of search sites in a group. Now edit your search query. Now 'Search' button gets enabled.
Search begins now.
Once the search begins, a result window shows up. If you have selected search sites from a group created under the 'Search Engines' category, then the window will have 4 columns - 'Name', 'Relevance', 'Site' and 'Search Engine'. Depending on the query given, each search site will return some results. Some search sites will give results according to the relevance. Some do not return relevance.