Sending a single message to a member or couple of members should not be a problem at all, however if you want to send something to all members, then I'd suggest to add an extra layer to your messaging system to implement a notification-like messages as well.
You will lose the message status (e.g. read, unread, etc.) however I think they should not be really important in that case.
I suggest to add the logic that if there was a message assigned to for example UserID 0, then show it to all members. Of course you need to delete it at some point, so it's not disturbing the members or you can have a different styling for this kind of notification/messages as well - for example they might stuck above the inbox, etc.
P.S. It's not answering your actual question, but I thought it's worth suggesting an alternative approach also.