Make it as easy as possible for people to join you mailing list. Put an opt-in box on every page of your site.
Most sites are NOT single page sites. They consist of multiple page. Not all of your site visitors are going to land on your front page. Many will enter through one of your other site pages.
The visitors arriving through your other pages may never even see your main page. So, make it easy for them to opt-in by putting a opt-in box on every page of your site.