To re-add opted out contacts to a texting campaign, contacts have to text one of your Keywords to your short code. If you've opted-out yourself, you have to do the same thing.
To find your keyword, you want to login to your EZ Texting account and go to Keywords & Textable Numbers in the left navigation. All accounts let you have at least one demo keyword that can be used for your contacts (and you) to opt back in to a texting list. Demo keywords start with the letters "ez" and are completely free to use.