The Groups in MoI is a great idea but it looks like a little confusing for many users here including me.
I have been using ArchiCAD for years.
For me the grouping in ArchiCad is the most intuitive and user friendly way of dealing with them.
Particularly after I had to deal with them in AutoCAD, where as most things related to editing is a nightmare,

You select objects, then Ctrl+G - group them.
And one command is crucial here - Suspend Groups (Alt+G).
After you execute it, all groups are open and then you just can select any object in particular group, then Shift+Ctrl+G and this object is not a member of this group any more.
And now you can make any changes in all groups you want - adding (Ctrl+G), deleting (Shift+Ctrl+G).

Alt+G and groups are closed again and all changes for just edited group or groups are implemented.