To build a list online, it is customary to offer a free guide or opt-in in exchange for an email address from potential customers. The most important feature of this particular content is that it provides value upfront.
If you are marketing a course, this guide would be an excerpt from that course so that your prospects can get an idea of what’s inside the course. If you are marketing a book, your opt-in might be an excerpt of Chapter 1. Also, the best guides give potential customers some kind of small result or win.