There are several ways to search:
Sort By Username
If you want to find all members whose usernames start with a certain letter, then you may select the letter from the top line.
Sort By Column Headers
To sort the memberlist ascending, you may click on the column headers like "Username", "Joined", "Posts", etc. If you click on the same column again, then the memberlist will be sorted descending.
Search for Members
With the "Search for members" search tool, you can search for individual members. You must fill out at the name field. If you don't know exactly what you are looking for, you may use the asterisk symbol (*) as a wildcard on any field. (e.g. know* will find knowall and anyone with a similar name or email address, if you include those in your search)