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.
Wondering what a keyword and short code are? If you've seen a sign in a store that says something like, "Text 'Deals' to 474747 to receive news updates," you've seen keywords and short codes in action. "Deals" is the keyword and 474747 is the short code.
When you sign up for Bitovation Corp, you will automatically be assigned to a short code. 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. You can find your short code by clicking on My Account or at the top of the Keywords & Textable Numbers page.