The commands to manage groups are on the Group menu.
The Group command combines a selection set of objects or groups into a single group.
The Open command lets you ungroup a group temporarily, and access objects within a group.
The Close command regroups an opened group. For nested groups, closing the outermost group object closes all open inner groups.
Ungroup separates the current group into its component objects or groups.
The Explode command ungroups all objects in a group, regardless of the number of nested groups, unlike Ungroup, which ungroups one level only.
The Detach (or, in Scene Explorer, Exclude From Group) command detaches the selected object from its group.
The Attach command makes the selected object part of an existing group.
