I’m looking for a way for only one person to use a command, but I do not know how or what to use to do that command
You can use $(eval) to check for a certain username:
!addcom -cd=5 !testcommand $(eval '$(user)'=='specificUser'?'[command response here]':'$(user), you cannot use this command!')
specificUser with the person you want to be able to use the command and replace
[command response here] with your own command response. And of course feel free to change the command name to whatever you want.