Groups
A group is a selection you keep for later. It is the loosest way to organise, and the only one that stays in FlexViz.
Select objects and press Ctrl+G to make one; the same key takes it apart. Click a group in the Groups panel to select it again later. Clicking one member in the plan selects just that member; Select group on the right-click menu takes the whole group.
An object belongs to at most one group. Ctrl+G refuses a selection that contains something already grouped and names the group in the way, so ungroup that one first. The Groups panel only appears once the scene has a group.
A group asserts nothing physical, which is why it can mix fixtures, truss and props. Two other things in the editor make objects move together, and they mean something different: coupled truss move as one because they are bolted, and a fixture mounted on a truss moves with it because it hangs there. Those are facts about the rig, not about how you chose to work.