Hi M

Thank you for reaching out to us.

If I understand correctly you want to sort the table alphabetically? Or would you like to filter the table by the first letter of the users name?

I'm afraid I can't see any attachments or related links so I can't a clear picture but please have a look at the link below that shows detailed information on how to sort wpDataTables 

https://wpdatatables.com/documentation/table-features/sorting/

I hope this helps, do let me know if you need further assistance.