Thanks Matt for your excellent advice, which is perfect.
By the way, I am answering your question:
I had added Source Dialog plugin from CKEditor using Add Button. But now I see that I didn't have to do that because Invision community is smart enough to show(or add) the Source button when a user has permission to post HTML.
I did not know that. Thanks for your strong support.
On top of that, I also added Table of Contents plugin from CKEditor using Add Button and it works well under the permission.