didnt know about this option, and most of my friends didnt either so I am sharing it with you, just to bring light to it being there.
If you Hit the ESC button, go to Options > Gameply on the right hand column at the bottom is an option to switch your Action bar to "Elective Mode". The tooltip reads " Enable full customization skill selection and placement on the action bar".
What this does is allow you, if you want to be able to use multiple abilities from one group, instead of one ability from each group as is the deafult.
You do this by turning on Elective mode,then right clicking the button you want to map. The screen will pop up with your abilites, you will now notice arrows on each side of your abilities that allow you to flip through the ability sets. You can now drag and drop the ability you want on to the action bar.
Hope this helps some of you that didnt realize this option was available and thought you were locked into using 1 ability from each group like I did