You can use the Search for a user and enroll as a student section to search for a user by username, or last name, in a specific LON-CAPA domain, or in an institutional directory, if the domain has access to an institutional directory search service. You can also click the Select User link and a window will open with a list of all the users in the course.
Note: If you know the exact username, you can either use a "username is ..." search in the search section, or type the username in the textbox in the Enroll one student section.