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I would pull it out as soon as you're seated to signal to the interviewer that you have it. Just put it on the table between the two of you, but closer to you, when you pull out a copy of your resume to give to the interviewer. If it's a large portfolio, keep it next to you on the floor propped against the table leg.
If they don't ask about it, you'll probably find a point in the interview where you're talking about projects or work you've done, and you can say, "I have my portfolio if you'd like to look at it."