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.