Organic-Groups multisite architecture
Honestly, I think that the module Domain Access would meet your needs more than Organic Groups would. A friend of mine recently created a site matching exactly your criteria.


  Module Package includes:
  
  Domain Alias 6.x.2 and higher only by bforchhammer. Allows you to
  specify subdomain aliases and domain name patterns for domain entries
  so that multiple hostnames are matched on a single domain entry.
  
  Domain Configuration -- Allows you to change select system variables
  for each subdomain, such as offline status, footer message and default
  home page.
  
  Domain Content -- Provides a content administration page for each
  subdomain, so that affiliate editors can administer content for their
  section of the site.
  
  Domain Navigation -- Supplies a navigation block with three themes.
  Creates menu items for each subdomain, suitable for using as primary
  or secondary links.
  
  Domain Path -- Enables custom path aliases per domain.
  
  Domain Prefix -- Provides a user interface for enabling select
  database table prefixing for use with subdomains. The Drupal 5 version
  is not fully functional on pgSQL, but Drupal 6 version is.
  
  Domain Settings -- Exposes domain-sensitive variable settings to most
  configuration forms. Requires Domain Conf.
  
  Domain Source -- Allows editors to set a primary source domain when
  links are written to content from other domains.
  
  Domain Strict -- Forces users to be members of domains in order to
  view content. This is a sample extension module that uses the API.
  
  Domain Theme -- Allows separate themes, theme settings and colors for
  each subdomain.
  
  Domain User -- Allows for the automatic creation of subdomains for
  registered users.
  
  Domain Views by mrichar1 and nonsie -- Provides Views integration for
  Views 5.x.1 and 6.x.2 or higher.