data validation - or function for text

You can use data validation that permits only one of there names, not a drop down list, but ... you question has no logic, maybe you should explain a why so it makes more sense.
If it's the acutal username, that just fille that cell with the active username when the file opens.

U
se conditional formatting for Cell A1 as an alert

=AND($A$1<>"andrew",$A$1<>"trudy")

Format as Red

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