The Group option is used to group individual parts. This feature is mainly used to make the manipulation of parts easier as it can reduce the number of individual parts which may need to be selected.
It can also be useful if your layout frequently uses an identical combination of parts with the same configuration. This may include 1/4 or 1/2 curves, or a repetitive pattern of roads or other baseplates. To copy the group you would only need to click on one part within the group instead of several individual parts.
To group a combination of parts you must first select all the parts you wish to have in a group, then right click on one of the selected parts and select "group" from the menu, or hit CTRL+G. To ungroup one must select on one of the parts within the group with the right mouse button and select "ungroup" or hit CTRL+U.
You can also have groups within a group. To do this, select an existing group (we will call this group 1), together with individual parts you want to include in the new group (group 2), and apply the "group" function as mentioned above. If you select any of the parts or group 1 and chose the "ungroup" function, it will ungroup group 2 but leave group 1 intact. In order to ungroup group 1 you must apply the ungroup function again.
The grouping function works exactly the same way with Parts, Texts and Rulers. Moreover, this action can be undone and redone, see the Undo and Redo section for more details.
It is possible to save a group of Parts in the Part Library (through the File > Save Selection in Library... menu) for an easy later use. For more information, please read the How to save a group in the Library section.