First 100 users, meet them one by one wherever you can (forums, friends, ex-coworkers) and call them to talk to them and help them out with onboarding if needed. Straight up cold outreach and warm intros. Next 500 will most likely come from referrals if these first 100 users are happy.
Apply this logic to the jump from 20 to 100 if it makes the task less daunting for you.
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