Before you grow your customer list you have to decide what kind of contact info you want to capture. This depends on who your audience is and your familiarity with marketing through different channels:
Email addresses – Best for those people over 30. More people are comfortable sharing their emails so you’ll grow your list faster and larger.
Mobile phone numbers – Best for the under 30ish year old customer, or if your information is very time sensitive and your customers will appreciate receiving it on their phones. You may receive fewer sign ups but each customer is assured to view your offer.