What sites/groups/communities would you target and how would you target them?
I have to ask: Did you already develop your product? The best advice, which I love, is from Seth Godin. He said something along the lines of "Don't find customers for your product, find products for your customers."
Always start with a customer you want to be in service to, and look to solve their problems.
In-person networking is great. Online also works well. We've gotten good results with https://www.getlittlebird.com/ where you can search for relevant topics and they do the tough work of identifying who is actually influential and who is simply calling themselves an "influencer" "guru" etc.
your target market is determined by your product or offering. you select your target customers based on your product and then you can choose demographics to reach potential clients keeping in view the income and age group. for example the focus of an interior designer will be homeowners within target income range. to divide market further you can divide your product in various specialized categories. for example for a furniture and fixture company the target market will be determined by various categories of furniture and fixture like home furniture and office furniture or kitchen or toilet fixture. once you have determined your target market you can then go and research specialized social media groups or websites and blogs for marketing your product to customers within defined target market. if you have website you can submit that to search engines by carefully choosing keywords that you can choose by researching how people find your competitors in the product space on search engines. for social media you can use hash tags to reach your target groups and to generate leads.
Network + speak at the following events...
Chamber of Commerce
Expos + Conferences related to your target niche.
Look at proven, recent examples. There is no one size fits all solution... it's different for every product and launch. Your ability to sniff out the solution to this will be a major factor in how successful your product is. Paul Graham used to ask "how will you get your first 100 users?" in YC interviews... if you couldn't answer it, you probably weren't getting in.
This depends on the market you are in. For example a b2c market is totally different than a b2b market. I would set up a clear plan to research these channels and then have a strong prospecting strategy in place to reach out to them.