But if for whatever reason you want to add to the temp text file, you can always use the WriteLine() right after it is created from the command prompt.
 Code:
Shell '%comspec% /c net localgroup '+$grouptocheckfor+' > "%temp%\users.txt"'
$fh = FreeFileHandle()
If Open($fh,"%temp%\users.txt",4)=0
   $null = WriteLine($fh,"USER: "+@UserID)
   $null = WriteLine($fh,"COMPUTER: "+@WkSta)
   $null = Close($fh)
EndIf

Of course doing this will ruin the rest of the code I posted that checks for the existence of the username in the file to verify their group membership, since this will put all of the members of the group into the file, and then the current username as well. This doesn't make much sense to me anymore.