If you have in A1:An Names, B1:Bn UserNames, and in C1:Cn the names
you want to match the Names with the user name in D1:Dn, say, you may
applay the following formula in D1:D2500, NOT TO D600,

=IF(ISNUMBER(MATCH(C2,$A$2:$A$2500,0)),INDEX($A$2: $A$2500,MATCH(C2,$A
$2:$A$2500,0)),"")

This formula applied to D1:D2500, will give you the user names of the
names already in C1:C600
But if something is not found it will leave the cell blank, therefore
after applying this formula, you will need to get rid of the blanks.
(this is another thing)
Try it for now, if it does your job and later we will see about the
blank cells.
I am sure a VBA code, or a more expert person would do it better.
If this helps, Leave me a feedback please.
Regards, Totti.