Ask them for feedback or their email, or just float the idea past people in a post. If they readily want to join a waitlist or check it out usually people will comment. Then it's up to you to create enough of a good UI and product etc. that when you send them a link they actually want to use it. Otherwise rely solely on comments/feedback. Here's an example thread where I did this:

https://www.reddit.com/r/SideProject/comments/d0xp35/i_made_...

>> continuing conversation/building relationships

When they ask for a feature, build it immediately if it seems like something smart to build. They will think you are a god because people are used to crappy customer service where they don't even get a response for days. So instead of waiting days to respond, build the features in a few days and respond that you built the feature. Then ask them nicely for a review and you will get 5 stars.

> feedback for each release

Ping the people who you know are power users. Also email every single new signup and ask how they like it.

Happy to provide personal feedback on your idea here too.