The plugin looks good - I see something we don't have in our specification / namespace yet:

From the Site concept:

<sioc:has_group rdf:nodeID="authors"/>

I need to think about this again, as before we were using a distinct property name to link two concepts, so that the property isn't biased towards either concept. For example, Forums are a "container_of" Posts (rather than a Forum "has_post" which is a Post), and Sites host Forums, etc.

So what is the relationship between Site and Group? :) Nothing obvious pops into my head just now, but there is something.

Maybe Group is linked to Site via a Role, Group -> has_function -> Role -> has_scope -> Site.