Of course it can't promote them. All of the best groups are secret, and with good reason. They are spread via word of mouth which allows group admins to vet new users. Facebook even added a flow to support a "before you join, submit your answers these questions" workflow.

Maybe that's not where the money is, but these private groups are so sticky that I don't currently see any alternative. People already know how to use Facebook, they don't know how to use Discord or Slack.