By the term “Site Group”, I mean an entity, which contain more than one site under it. Changing settings of a “Site Group”, will affect all the sites assigned to that particular “Site Group”.
These settings can be enabling plugins and themes, time zone, permalink style, etc.
Although a site is assigned to a “Site Group”, it should be possible to change those setting in each site’s “personal” admin panel again, without effecting any other site in the group.
More at my blogpost (Including graphics!): http://blog.budhajeewa.com/suggesting-site-groups-for-wordpress/