The Group option is used to create a new group from selected Actors. If none of the selected Actors are already in a group, only Group will be available. Choose Group to group the selected Actors.
Grouping is non-destructive in that it never destroys existing selected groups. Any number of Actors or groups can be included in a selection when creating a group. For example, if two Lights and an existing locked group of two Static Meshes are selected and then grouped, a new group will be created with the two Lights and the sub-group of Static Meshes under it. If any selected Actors are in unlocked groups, they will be removed from their old group and placed in the new one.
Once a group is formed, green brackets appear around it, and its pivot point is automatically set to the center point of the group.