Attaching Guard posts to deck joists
If this is an existing deck, the rim joist already in place is the structural member. If you are worried about compromising the strucural integrity if the new outer board; you might, but you shouldn't worry about the overall strength of the deck. If you are worried about having a wobbly handrail, the best you can do is attach a post to the rim without attaching the outer board. Then give it a good shake. If it wobbles more than you would like, then bolt through the outer board, too. I would add blocks in between the new rim, and the old rim. Here's a diagram.