Enhances the Post "Visibility" options within the edit post page to allow specific users or groups of users access to the post.
Initially this was developed for use on my own site to allow proposals I created for clients to live within my WordPress site as a custom post type but one that (even if someone stumbled across the URL) they wouldn't be allowed to see. But the uses are already going beyond that for me and I'm sure there are many implementations that I haven't even thought of.
Within the edit form for any of your post types (see below for configuration options that will determine which types of posts get this option) there is a new meta box that allows you to indicate what type of access controls you would like to enforce on that post. If you choose "By Individual" then you are given a list of all the current users for that WordPress installation. You can check as many of these checkboxes as you'd like to individually give those users access to that particular post. Similarly you can choose "By Group" from that same drop down and you will be presented with a listing of groups that users can be assigned to for more reusable combinations of users.
As mentioned above, each post can have one or many user groups that are given access to a particular post. These groups are maintained through the "User Groups" forms that live within the "Users" menu in the WordPress admin. You can create as many user groups as you'd like and assign users to whichever groups make sense. There is no limit to the number of users within a group and there is no limit to the number of groups a given user can be in. These groups can make this a great tool for websites for organizations that have committees or teams that need access to some information but that information is not public. A user group can be setup and re-used over and over. And then if a member leaves (or is added) it only needs to be maintained in that one group definition rather than adjusting all of your posts.
These admin forms have been built with all the WordPress admin tools and structures so they feel like they are part of WP Core. Easy to browse through, filter results and maintain all of your groups and their members.
There are just a few configuration options for this plugin available through the Settings menu: