There are three methods to add subscribers to your list.
1) Keyword Opt-In (most popular)
Subscriber can opt-in to your list on their mobile phones by texting your list'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.
Subscribers may be added via import. Anyone opted in must have provided an opt-in to receive messages from your organization. Post-import, we strongly encourage that you send a manual version of the opt-in message to let subscribers know they have been added to a list.