There are various ways in which you can grow your own opt-in email list. You have to ask for their email addresses when they visit your website and in return provide them with some deliverable like e-book, white paper, etc.
Or just promote your Newsletter that you can use to promote discount and news about your product.
A pop-up window asking them to share their email address should help build a list of subscribers. You can have a sidebar that will ask your blog visitors to sign in and share their email ids. Ask for their permission to send emails and promotional items.
