question

I think I see what's happening. When you right-click on the grid itself (and not an object), you get the option to select all the members from different groups. This is the menu that is showing in the screenshot you sent. When you right-click a 3D object, you instead have the option of adding that object to a new group or an existing group. I'm thinking there's a possibility that when you right-clicked, you clicked between the chairs and accidentally clicked on the ground (grid), and not the waiting room chairs.