Is there a feature or plugin that will allow me to have multiple authors on a site. An author must be able to specify which user group they want to be able to read the post. Ideally users will be able to belong to more than one group.
Topic multi-author user-roles plugins Wordpress
Category Web
This is a multi-part question, so I'll break it down.
You're already able to have multiple authors on a site, with Wordpress's default setup. There are several ways to restrict the categories they publish to. One is through categories. This would rely on the honor system, and on them selecting the correct category. It's also the easiest to implement (in fact, it might not need any implementation). So whether you want to go this route depends on the level of trust you have in your authors.
If your trust level isn't that high, you can do a more formal author restriction via custom post types. These can be created manually, which is how I've done it. They can also be created with plugins; I've never done this personally, but here are some options: https://wordpress.org/plugins/search/custom+post+types/
Once you've created the custom post types, you should be able to use a plugin like User Role Editor to limit certain authors to only posting to certain post types: https://wordpress.org/plugins/user-role-editor/
So that's all on the content creation side. As far as displaying different results to different users, the complexity depends on if this is a public or private site. If it's public, then just show users the feed for that category or custom post type, and you're done.
If it's private, you might do something similar, but restrict access to those pages through a membership plugin. I used Cart66 at an old job; it worked all right but there are better dedicated solutions out there now.
I hope this helps! :)
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