This comes up with some frequency. A single link that you could share at a conference or on a protected web page, for example, to let many people get logged in quickly would be quite useful.
I made GitHub - pfaffman/discourse-user-creator: Create an activated user, optionally assigning to group · GitHub, though. You can use it to create accounts that are activated and have passwords assigned and are in a group. You could create them with bogus e mail addresses. That would get people in. They’d then need to change their email address and password, and perhaps their username, which is a fairly complex set of steps
But you can configure a group to let people join freely, so maybe that would solve your problem.