Say you have a table like below:
Create a variable as: Index =Left([Employee];1) and create a separate table with this object.
Right click on Index column and go to Linking > Add Element Link.
Select the employee table in dependency section.
Now click on any alphabet on the index table to filter the employee table.
Drawback: All alphabets (A-Z) may not be visible in the index table. Only those alphabets will be available for which some employee name exists starting with that letter.