Out of Office Message... that broadcasts to a SPECIFIC team?

If your meetings aren't private but are to the group all the time, you can schedule them in a channel then join the meeting and add the note in chat, because it's a channel chat it will be persistent and show the chat to them when they Join the meeting.

Outside of that, your best bet might just be to invite them to the meetings individually and then cancel them so they no longer show in their calendar to join, this way they know it's not there any longer if it's missing.