Private Lines
There is another way that may or may not work for you, but needs to be set up by the GM.
I use Language groups to set up private categories, usually for Lore Skill training. That way I can send one PM to everyone trained in a particular skill the same message/bit of common knowledge. Saves some work when giving out specialized background info.
It can be used for other things, of course. Private Telepathic groups, or, really, any specific category the GM wants to set up.
Of course, it's still obvious that it's being sent to a group, just not who is in the group. And it could well be a group of one person, even. But, as time goes on, players are likely to learn who is in the group through play.