You'll have to have the contact text in from their own phone to be added back again. We don't allow you to opt someone back in for obvious reasons.
Long code opt-in
On a long code, simply have your contact text the word START to your number to be added back.
On a shortcode, you'll need to have them text a keyword in your account. If you don't have a keyword, you can add a free one under TEXT MESSAGING > MOBILE KEYWORDS.