Use it for questions related to the Group module, not the Organic Groups module.
The Group module allows for creating arbitrary collections of your content and users on your site, and grant access control permissions on those collections. It is available as of D7, and has a D8 version also. It is designed to be an alternative to Organic Groups.
Organic Groups allows content itself to be groups, which isn't always what people want. It relies on an entity reference field to keep track of the ties between a group (node, term, ...) and its content (node, term, user, ...)
Group instead creates groups as entities, making them fully fieldable, extensible and exportable. Every group can have users, roles and permissions attached to it. Groups can also act as a parent of any type of entity.
The Group module integrates with various other (popular) modules, including: