Is there any equivalent to a Unix group in Windows?
If so, where is there documentation on its structure?
There are default groups that come with the base OS before Active Directory (AD) becomes involved (where the host has joined an Active Directory domain etc.)
These are documented as well as the default rights assigned (for more recent versions of Windows) here:
Once AD becomes involved it can become much more complex depending on the environment. I'm assuming that your query was more along the lines of those groups that belong to a single host in a default install.
There are OUs and groups in AD, but there isn't a concept where every user has a user:group. They are more like labels that can be applied to users. One group is administrators, which, as imagined, gives administrator access to the users in it.