You can send a PM to the Registered Users group, then everyone on your board will receive it.
First make sure you have the permission "Can send messages to groups" on Yes for the Administrators group, in Groups' permissions under the Administrators tab.
Then go to the Users and groups tab, Manage groups. There edit the settings for the Registered users group and check the option: "Group able to receive private messages". Now you can PM the whole Registered users group in one time.