Ideally, you need to validate your product on whatever channel you intend to use. So if you expect to get signups mainly through your website, you need to test

Ideally, you need to validate your product on whatever channel you intend to use. So if you expect to get signups mainly through your website, you need to test that channel as early as possible. Eventually you'll be getting hundreds (millions?) of signups through your website. So getting just a dozen for your beta list should be easy. If it's not, then you need to either pivot or find a better channel. But if your website isn't the way you expect to gain users, then try a different channel. And I don't think Slack is a good example since Campfire and Hipchat were already in use many years earlier.