What is what?
Groups: a group of people doing something. This can be anything from running a whole building, a kitchen, a organizing committee for an action... Groups can have a physical location, but they don't have to.
Events: anything organized by a group.
Location: the physical place where something takes place. This can be a full postal address or just a dot on the map. It also can have an 'unofficial' name under which it is generally known. One or more locations can be attached to a group or event.
Users: you. The individual people who are part of one or several groups. You need to make an account to add a group or a new location.