I was just looking for this answer, and here it is!
Truth is, I almost NEVER use the public messages, so it didn't cross my mind. I'm running into this for profiles where LOTS of merging has been going on, and I need to communicate with ALL the profile managers. I've been copy/pasting the same private message to each one individually!
Still, while this works, I'd like to see a way to private message ALL profile managers for one profile added to Wikitree. I don't feel that every discussion should be on the public bulletin board.