You want to first identify the type of community groups that you want to target by building out your Ideal customer profile. It's a great start that you have a clue who you want to target. After that, I would go with cold emailing. It's the most efficient and cost effective way of gaining your initial traction. You can do the list building and cold emailing yourself in the beginning or you can outsource it to a company like http://inspirebeats.com

InspireBeats will help you build a list of targeted communities, find their contact information and even do the outreach for you to warm them up about your product.