I want to create a community where each person will collaborate with max 1-2 persons. This means that the community will have groups of 2-3 people. The content they will create, must be available only to them. they will have to be able to tag, search e.t.c
This will be the 95% of the websites content.
Additionally i will have to provide custom functions to this with the biggest being to be able to export the content locally to the user in many formats.
What is best suited to do?
Create this in a forum or in a CMS like Joomla!/Drupal ?