We are currently working on Alfresco (the Open Source version) to add Group support.
We have not yet decided if release the source code as Open Source, but what happens if we do?
As far as I know, there's already an implementation of Groups in Alfresco (probably quite different than our), but only in "Commercial" versions; we have two probable scenarios:
1) The patch will not be accepted by Alfresco. This means that the Alfresco Open Source edition is just a "light" version of the commercial edition, and that Alfresco is not really interested in creating a developer community.
2) The patch will be accepted by Alfresco, and will become part of Alfresco "Open Source". The commercial version will retain the current Enterprise implementation. This is a better scenario than the first one, but this means that the (probably better) Enterprise implementation of groups will never see the light on the Open Source edition. This means also that the migration path between Open Source and Professional/Enterprise edition will be much tough, as they have incompatible group data.
This is not a flame post: we like a lot Alfresco, and we want to help its development too, but we need a better understanding of the "community process" around it.