If you use the Drupal organic groups approach, then each organic group has an identical functionality set (same forums, blogs, events functionality--- the actual posts forum posts, blog post, events are just for the organic group you specify and could be shared across multiple groups).
Something like CivicSpace on Demand (which gives you a full Drupal install for every site) will allow each site to determine what functionality they will use.
You can build sites like these by downloading and configuring Drupal/CiviCRM yourself or you can hire any number of consultants in the community to do it for you.