I apologize for this question, as it must have been answered before. Just can’t find that answer.
I want to create three groups of people who use my Discourse instance. But I want members of a given group to ONLY see the posts, messages, categories, tags, members, etc., from their own group.
How do I set this up? Or do I need to have a separate Discourse instance for the kind of walled-off group I’m describing?
Thank you!
You can make specific categories accessible to individual groups, but you can’t prevent members of group A from seeing members of groups B and C listed in /users or messaging them specifically.
PMs can be disabled site-wide though.
Your use case is too ambiguous to really make a recommendation, you’ve not said who these users are, if they’re coming from a different system, or anything really.
I apologize, but the use case is private. You’ll just have to trust me that it makes sense for my organization. 
Thanks for replying, though!
If you have essentially three separate fora that you want to run on the same server, then you can use multisite:
Ah, thank you, Anrdrew!
You can turn off the user directory though?