I need to set up access to media files with the following structure: A media folder is designated as belonging to a specific member group. Then, a sub-folder below that needs to be available to a subset of members from the parent's member group. Any thoughts on how this can best be accomplished? I'll render the actual file download links with a user control, but how should I set up this access control on the back end? I need a member-group picker and a multiple member picker. Do these exist anywhere?
Never used it, but there is a Filtered Member Picker project on our.umbraco. You may also be interested in the Protected Media package. I can't attest to their quality, but they may help guide you.
This feels like a custom section in the admin pages where you make a page(s) to manage this nested hierarchy. See the videos on Umbraco.org for how to create a custom section.
You'll write the aspx page(s) for the custom section like you would in a vanilla ASP.Net site and probably store your authorization information in custom database tables.
You'll also write a business logic class(es) that supply this information to a web user control you write. The web user control(s) are wrapped inside macros and used in a template. They'll expose the autorized content to the given visitor.
Its not a tiny job, but you should be able to get close to the end result in a day's time if you're really familiar with Umbraco; then polish it up from there. There is no simple configuration for this, you might find a 3rd party add-in, but that's doubtful. If you can solve this challenge using standard ASP.Net, then you can easily add in a custom section to the Umbraco admin pages and get a similar feature up and running.