There are two methods to add subscribers to your campaign.
1) Keyword Opt-In (most popular)
Subscriber can opt-in to your campaign on their mobile phones by texting your campaign's unique keyword to the 5-6 digit phone number (short code) assigned to your account. To setup your keyword, click here.
2) Website Opt-In
For instructions on how to add subscribers through a website, point of sale, etc., check out this post.