Settings in administration panel:
Overall -> Configuration -> Private messages:
max receivers: 0
messages to many users and groups - yes

Users and groups -> Groups -> group management -> adminnistrator group settings:
max receivers for one pm: 0

Users and groups -> Groups -> group management -> registered users:
max receivers for one pm: 1

Administrators should be able to send pm to more than one user. But, they can send only to one user - according to registered users group rights. Administrators should override users rights.