The best way to search for members on RunnerSpace is by using the top search bar.  Type in the persons display name and the search results will instantly pop down. You can now put your mouse over the "Members" area and this will filter out all the members in the search results.  Scroll down that list until you find who you are looking for.  Some names have MANY results, so you many need to press the "load more" button a few times or revise your search word.

Below is an example of a search for "Ross":