The way that worked for me was to DM people on Twitter or Reddit. You can identify your target users based on who they follow. Don't spam or send generic messages. An example I can think of is the following:

"hey xyz,

Saw you following Foam Freesearch. I am building a product in the space between networked thought and thotworked net.

Right now, we are doing a Private beta testing with a small number of users who are interested in this space. Therefore, reached out to you, in case you are interested to join.

We have limited slots, Lmk if you want in. Kudos "

I got around 50 users to try out private beta for Primer (https://primerlabs.io) that way.

My business partner did a PhD in the space and surveyed a ton of folks. She did all this before a line of code was written. When we had something to show, we went back to this pool of users and asked who'd be interested in beta testing what we built.