That's a great question! It really depends on a few things:
-What type of crowdfunding are they looking for? Rewards-based, donation-based, or securities (equity or debt)-based? Answering this will make it easier to understand what type of platform to look for.
-What is their goal with the campaign? From a fianncial standpoint, you may want to consider what the unit price will be per product and the demographic that can afford that product.
-Is there anything about the product or service that would limit it to certain demographics or that you have to have another product in order to use it? For example, if it's a smartphone mobile app, you may want to consider limiting your exposure to those with smartphones.
To get a little more in the weeds on actually constructing an audience, I'd first start with asking yourself what your customer avatar should look like and expand from there. Maybe you're selling running shoes, so your first thought is only appeal to runners. While the may be a very immediate market to address, consider other things outside of just "enthusiasts". Think of how many people purchase running shoes that just want to be active, wear them for fashion, support that bradn, etc.
Next I'd look at the success of response rate and product turnover from your current social media following and email leads. Their praise, frustrations, even demographic make-up will not only tell you how to better market your product but who you may/may not have left out of advertising to.
Then I'd start constructing these audiences in paid advertising, particularly Facebook and Instagram and start testing ads to these audiences. After about a week's time you'll start to get a much better idea of what does work best.