Groups in ArchiCAD

Thanks to shoegnome - link added.
Repeated here FYI:

1. The enable/suspend groups button should always be visible.  If the button looks like the image below, then groups are enabled. Otherwise they are suspended.
2. It’s a good idea to have shortcuts for enable/suspend groups, group, and ungroup. This is what I use:
   command+G: groupoption+G: enable/suspend groupscommand+option+G: ungroup
3. When groups are enabled, you move the group as one unit. When groups are suspended, you can move individual elements within the group while maintaining the existence of the group. Ungroup means the elements remain, but the group is gone.
4. If groups are enabled and you group 2 (or more) existing groups (lets call them A & B), the subgroups remain within the larger group (lets call this C). What that means is that if you ever upgroup the new group (C), groups (A) and (B) will remain. However if groups are suspended, all groups are suspended, not just the outer most group. This means to move group (A) in relation to group (B) you either have to ungroup group (C) or suspend groups and move the elements individually. It would be awesome to suspend group (C), but keep groups (A) and (B) enabled. But there are more important things in life. Note, if you make group (C) from groups (A) and (B) with groups suspended, the subgroups will be lost. Note, when groups are ungrouped while they’re suspended, they will all become individual elements, regardless of whether they were in nested groups.
5. If groups are enabled and you move/rotate/copy/mirror a group, all the elements are effected, even ones on hidden layers. If some of elements are locked and groups are enabled, the entire group can still be moved, rotated, etc. If you have groups where not all the elements are visible at all times, make sure you know when groups are enabled/suspended (see point 1). Otherwise you will be angry and annoyed.
6. Groups can span multiple stories. You can’t group 3D objects in section/elevation, but you can group them in 3D. So if you group elements from the 3D window, you can have a group that contains a wall on the first story, a slab on the second story, a column on the third story, etc. If groups are enabled and you move part of the group on one story, the rest of the group on the other stories moves as well. In version 14 (maybe earlier versions as well), when you move/rotate/etc. the group in plan you will see the ghost of the grouped elements from the other stories as well.
7. If you group 2D and 3D elements in plan, then group multiple 2D/3D groups in the 3D window, you will be able to have 2D elements within the multi-story group. If you then suspend groups, delete all the 3D elements (in plan, section, elevation, or 3D window), and then enable groups again you will end up with a multi-story group containing only 2D elements. Try it.  Similarly you can group two elements then delete one, leaving just one single grouped element. Such an element cannot be edited without suspending groups. This could be as bad as it is good.
8. Grouped elements can be changed even with groups enabled, by way of parameter transfer (syringe).
9. Groups are represented by eight colors on the following order: olive, dark blue, burgundy, green, blue, dark red, wine, plum. Quite the smorgasbord and after that they will repeat! 
10. Grouping of 3D elements can only occur in plan or 3D. Grouping of 2D elements with other 2D elements can occur in any view or layout (including master layouts).
11. The following cannot be grouped: all dimension tools, all viewpoint tools (including cameras), zones, labels, doors, windows (including corner windows).
12.  Drag/rotate/multiple/mirror/etc. a copy of a group with groups enabled, the copy stays a group. Drag/rotate/multiple/mirror/etc. a copy of a group with groups suspended, the copy is no longer grouped.
13. Copy a group from one file to another (or within a file using cmd+c/cmd+v) and it remains a group regardless of whether groups are suspended/enabled in either file.
14.  Inserting a new node to a wall, beam, or line creates two elements. If the original element is part of a group, the new element is also part of the group. Use the split command to cut elements into pieces and everything also remains in the group.
15.  Unifying 2 (or more) polylines/lines: If the last element selected is part of a group, the new unified element will remain in the group (and also take on any other properties of that element–layer, color, etc.). If the last element selected is not part of an existing group, the new unified element will not be part of a group. This is actually more a function of unify than of groups. The final element to be selected is the element that all the others become a part of.
16. Linework/Fill Consolidation: This is a bit of a mystery to me. If all the linework or fills are part of one group, then the consolidated element will be part of that group. If there’s a mixture of grouped and non-grouped elements, sometimes the final object is part of the group and sometimes it’s not. And if there are elements from multiple groups, it varies on which group the final element is part of. I have yet to figure out the pattern. Maybe someone else can.
17.  Autogroup: If the Autogroup command is on, chained polygonal and rectangular elements (such as PolyWalls, PolyRoofs, etc.) will automatically be created as a group. Components of exploded elements will also be grouped. (#17 is straight from the ArchiCAD help menu and worthy of being on this list).