How to create landing pages and Main Menu based on taxonomy
This seems like the perfect use case for Organic Groups


  Enable users to create and manage their own 'groups'. Each group can
  have subscribers, and maintains a group home page where subscribers
  communicate amongst themselves.
  
  They do so by posting the usual node types: blog, story, page, etc. A
  block is shown on the group home page which facilitates these posts.
  The block also provides summary information about the group.


There is also a kick-ass set of tutorial videos from BuildAModule here that should help you set it up.