You just need to DIM the $list first.

Code:
Dim $list
$objDomain = GetObject("WinNT://"+@Domain)
$count=0
For Each $objComputername In $objDomain 
   If $objComputername.Class = "Computer"
      redim preserve $list[$count]    
      $list[$count] = $objComputername.Name 
      $count=$count+1 
   EndIf
Next
$null=messagebox(UBound($list),"Computer",0)


Works fine for me...