Dave Hill, Mark Aufflick, and myself implemented something similar to what you are describing (organizations are groups, using the membership groups rels mechanism, etc) on a site for pps.org that isn't yet public. I believe Mark was going to wrap up some of the code and offer it back to the community.
An interesting side note is that we allow persons to be members of a group and that we have a mechanism in place to transform a person to a user. We have a lot of content already on the site and it seems to work fine. Its was fairly easy to do. We also use acs_rels to map content (quotes and excerpts for example) to their source piece of content, multiple authors (can be any kind of party) to a piece of content, multiple funders to a case study, etc. After Mark did the work of getting a suitable version of a rels widget to work, this was also quite successful.
The code could definitely use some polishing, but it works well for our uses. It also has some customization that would need to be pulled out. Some of which is related to how we use the categories package to classify individuals, organizations, and content.
Mark, how far did you get with the releasing of this code? What stuff was included?
Dave and I have to get over a work deadline before we have time to wrap this up ourselves.
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