All site changes are made under "Manage Property", "Property Layout". You are able to add or edit groups or units there using the spheres and the edit buttons at the bottom right of the page.
There are specific group types depending on the use of the unit. The group types and their purposes can be found under the following link: What are the group and unit types?
The layout of the units is based on access and not actual physical placement of the units. In other words a common or amenity group at the top level of the property is accessible by everyone whereas either of these groups placed under a building or floor is only accessible by that building or floor.