When we go to Users > Search for users in the admin side, by default its sorting with respect to User ID and there is no option to sort with respect to username.
Learning Management System
To change the default order by when searching for users go to system admin -> Configuration -> search selectors. This will populate all the searches by the LMS. Expand the list to display all and then find on page (ctrl + f) and search "user" until you find the search selector labeled just "User". This is the configuration for the user search. Add the below configuration > and click Apply Changes
We would recommend adding it near the other searchResultColumns.studentName configurations though you can add it anywhere. This will make it possible to also be able to click the column headers to sort the search results to your liking
searchResultColumns.studentName.orderbyOptionRef=By Name Sort By User Name User Name , KBA , LOD-SF-LMS-ADM , Admin Tools , How To