Object Groups are the way to add more organization to your diagram. They allow to group several diagram objects to single Object Group. All objects in this group will be moved with group. You can also delete whole group with or without all its objects.
Here is an example of Object Groups usage. This is a sample diagram of "Sakila Database" available at http://dev.mysql.com/doc/sakila/en/sakila.html
There are 6 Object Groups created within this diagram to join tables and other objects into logical groups:
Stored procedures and functions
Object Groups are displayed as semi-transparent rectangles on diagram. Group's name is displayed in left-upper corner of semi-transparent rectangle. You can adjust Object Group transparency and color with Object Group Editor.
Font size of Object Group name is independent on current diagram scale. So you can see Object Group name even when you use small scales (for example 33%) to observe or move a lot of diagram objects simultaneously.
Object Group size rectangle on diagram is adjusted automatically to fit all objects attached to this group. You can also change Object Group size using resize markers, available when group is selected. You can also move group (with all its diagram objects) like any other object. And you can move diagram objects inside Object Group without any restrictions.
Object Groups are also displayed as bold frames on Minimap Navigator.
With Database Designer for MySQL you can:
Create Object Groups;
Edit Object Groups;
Add and Delete diagram objects to/from Groups;
Delete Object Groups with or without containing objects.