Is there a way to allow the sharing of user accounts and various module data across portals under a single installation?

We have a situation where we have a parent organization and a number of member organizations, each of which would have their own sites, users, etc.  We need to be able to propogate and share data between these sites.  How can this be done in DNN?

Primarily we need to be able to combine events calendars to do a rollup from the children to the parent, so as to provide a master event listing summary, as well as profile data (from a custom module we will develop) so we can have users manage their profiles in their local site but also have them accessible from the parent organization's site.  We will also have some memebers that will have external sites they maintain themselves so they need the ability to login to the parent site to maintain a profile, events, etc to feed into the combined system.

Any suggestions on how to do this would be appreciated.

I have done this for a big multiportal system that I created.  There's a vendor called DNN Masters which produces a module call Multi Portal User Sharing to share users across portals within a host.  Do a search on www.snowcovered.com for DNNMasters.