I have no idea how "Reddit + Fediverse" (i.e. Lemmy) works in terms of federating, but on Reddit, all you had to do was select an option to set your subreddit to private (only visible to subscribers).

It seems strange if Lemmy doesn't have a similar feature, for example, if you run a niche community and don't want random people giving unwelcome, ignorant, malevolent, etc. opinions. Also, in case Lemmy ever has enough users to be prone to "brigading" (groups ganging up on a community to downvote everything, harass subscribers, etc.), a "private" option seems like a basic and necessary feature.