@canuck78 is absolutely right. No point to notarize. As someone who practised law in Canada and was also a notary public by virtue of being a lawyer, I can say for sure that "notarizing" an invitation letter will do nothing. In fact, an honest notary will tell you that.
I see many times on this forum advice to get various documents notarized. Most of the time, it reflects a misunderstanding of the notary's role. A notary can compare a document copy to the original and certify the copy. That's useful and appropriate in some cases. The only way a notary can lend any strength to a letter of invitation or such is to put it into the form of an affidavit or statutory declaration, wherein you swear that everything you say therein is true.
Even if you swear or affirm the content of a letter of invitation before a lawyer or notary, I doubt the IRCC will give it additional weight. For one reason, the IRCC expects the invitee(s) to have sufficient funds of their own to cover all costs of their trip, including accommodation and food. You saying under oath that you'll provide food, lodging, money or anything else won't divert the IRCC's gaze from what the applicants bring to the table without regard to anything you promise to do. In my view, the only purpose of a letter of invitation is to support a genuine reason for them wanting to visit.
