"Show HN" (https://news.ycombinator.com/showhn.html) is a great way of getting a general (but very techy) group of people to try it out and give feedback.

Otherwise, if you're solving a specific problem in a niche, start hanging out wherever those people hang out, participate in the community and share your solution if it's applicable to the discussion. Make sure not to spam/self-promote a lot though, as it comes off as really tacky in most places.