So if you already have signups, then you should absolutely work with them to add referrals. Send them a free month offer if they give you the email of 5-10 of their friends likely to need your service. Or some variation of that idea. You should absolutely be sending them emails and talking to them regularly as well. Let's say you have 100 active users right now, I'd be calling the top 25 and talking to the people directly and getting their feedback. It doesn't mean you will implement exactly what they ask for, but the fact you picked up the phone and called them to talk and get their input will go a long way towards building loyalty.
If the people signed up are the right audience, e.g. they are engaged and using the product, then get some demographics on those people. If you didn't collect it up front, do a survey, or utilize a marketing company to get you data on those contacts (like Acxiom or other many other providers). This will give you a profile of the type of people, or companies if it is a B2B app. Then take that data and do some Facebook or Display ads that are targeted at a similar audience.
Also, if you have a free trial of say 14 days, offer to extend it to 30 days if at the time of signup the user gives you 5 emails (names) of people as a referral. Then you can solicit them.
If you are starting from scratch with an already built site, then you need to reverse engineer the demographics of who will be your early adopters and go find where they hang out, where they do their research and what is important to them to make a decision etc. Then you setup offers where those people are researching (or playing etc), and you get active in those communities. Also, good time to write some posts on medium and publicize them and start writing some of your own blog posts that you can point people to as well (again promote them places). In the end, this is a numbers game and it is about getting out there and exposing yourself and your product to as specific of a group as you can to find the right fit. Then you keep expanding the types of groups you target.
Hopefully that helps a little more. One other comment, Marketing is about doing a series of experiments where failure is guaranteed on a pretty regular basis if you are trying enough. Marketing is never about getting it right once and never looking back, clients are always changing, and you have to chase and find where they are and find a way to get people to reach into their wallet and fork over money to you. Failure in marketing is different then failure in say engineering some life critical piece of equipment, in both cases you will learn from failure, but obviously no one dies if you jack up a few marketing campaigns so the relative cost of learning is low.