[calorie]

Take a look at the private.php file near this...

// form for creating a new private message
if ($_REQUEST['do'] == 'newpm')
{

Scroll down from there, and there is a hook that you can use: private_newpm_blank

The query string should be like this: /private.php?do=newpm&u=101,102,103,104 (with only one u - but you might want to use something other than u like pmto or something).

Right above the private_newpm_blank hook is code showing how to populate check permissions and populate $pm['recipients'] so just use that as an example. You can clean pmto (or whatever varname you choose) and use it to fetch the user names.