are you already using mailchimp? do you have a contact or inquiry form? If so there is no easier way of increasing your list than to automatically add them after they submit a form, after all, they are already submitting their email so why make them type it again just to subscribe them?
Do you want to add them without give them a chance to say no (that’s mean but possible)? do you just want to add them only when they check a box (which could be checked by default wink wink) or maybe you want to let them choose between your weekly newsletter or your monthly promotions (which is a great way of increasing the quality of your lists), all of this is possible and configurable easily =).