[Corsec]

set_recipients (line 273)

Accepts a list of recipients names to create the touserarray field
boolean set_recipients (string $recipientlist, array &$permissions, [mixed $type = 'bcc'])

    * array $permissions: array for sending user.
    * string $recipientlist: Single user name, or semi-colon separated list of user names

Well, by the looks of this, the first arg should be a string of users to send the PM to. The second arg looks as if it should be an array for permissions, but not sure what would go in there. Hope this helps