an app to generate 'yahoo groups' features/functionality..

hi.. after a lot of fruitless searching via google, i've decided to ask here... does anyone know of an app that will allow a site to provide functionality similar to 'yahoo groups' to the users. i want to be able to allow users to be able to create their own 'groups' and to be able to have the user allow other users/members to join the group. i also want to be able to have the user be able to admin the group, with the ability to manage members/handle message posting/(up/download) pics/etc... i asumed that there would be a number of these kinds of apps, i haven't found any yet. i have found plenty of bbs/forum types of apps that try to pass themselves off as being in the 'yahoo group' area. i've also seen the drupal based civic app... thanks for any help/pointers/etc,.... bruce bedouglas@earthlink.net