Well, there are 8 possible groups, so I would expect 8 possible drive letters.
Code:

$arrDrive = "S:", "T:", "U:", "V:", "W:", "X:", "Y:", "Z:"
$FriendlyName = "S share", "T Share", "U share", "V Share", "W Share", "X Share", "Y Share", "Z Share"

Would there also be a good reason to stay away from the Z: share? Maybe in worst case there is still a W9x computer or so... I thought I read somewhere something...