I saw that on some YouTube clip. In example, I have several Barracs and I want to set the rally point for all of them in the same spot? Is there an easy way of doing that then manually setting the rally point for each building separately.
I am not able to test it right now, but shouldn't double-clicking on the building select all similar buildings, and then right clicking on the location you want to rally do exactly what you want?
Alternatively selecting each building you want to set the same rally point holding shift should, afterwards again right klicking to the location.
Alternatively, you could add additional buildings to a control group later. Select the first building, then ctrl+#. Then select the next building, and shift+#. You can use shift+# to add additional items to any control group.