I encourage you to read some answers of mine to similar questions asked on Clarity below.
The value of Direct Outreach: https://clarity.fm/a/2149
Understanding your customer acquisition costs: https://clarity.fm/a/611
Using Twitter for early outreach: https://clarity.fm/a/438
The most important thing to do is to qualify your users to ensure that they really are the people that really feel the pain for the problem you are solving. Offering them early access in exchange for a quick survey is a tactic I've used with pretty good conversion.
Happy to help talk through the specifics of your path forward in a quick call.
I am guessing you have a good idea of who your likely customers could be - maybe you have an idea of who your marquee customers could be, and a list of smaller companies that could be your early adopters.
The best way is to reach out to these companies and talk to them and sign them up as beta customers for when you launch.
Of course, you could also post on forums like Hacker News or forums that are relevant to your customers to attract their attention. However, I feel a real conversation is the best way to get a feel for who will sign up as an active beta user.
Reach out via people in your network, or request for a short call to get feedback from your potential customers and try to turn them into beta users.
I personally built our beta community when launching Vidyard (YC - Summer 2011).
Our model was built around signups on the Vidyard landing page that we'd nurture via e-mail campaigns and eventually add to the product (when we were ready) to stage growth.
I followed and engaged with everyone on Twitter that breathed a word about B2B Video and Video based marketing campaigns. My bio was designed to draw people to our landing page, where a portion of them would sign up for the "beta list".
The metrics worked out (ish) as follows:
100 follows a day --> 20 conversations on twitter --> 1 signup to our "beta list".
I then personally engaged with each signup to validate their need against the product we wanted to build. When ready to launch, we had ~1500 people on the list ready to try the product.
I spent ~6 hours a day focused on this.
Happy to jump on a call to discuss further!