Hmm... well, I just spent five minutes searching, and I can't find the relevant setting, if in fact it exists. Let me know if you can find anything on meta.discourse.group
...
Okay, yay! Found it.
Down side: there are some things to configure, and some gotchas.
One fundamental issue is that by default, users must be at trust level 2, "member", in order to start new topics. Thinking harder, this is probably a foundational pillar of spam prevention, and I'd be leery about adjusting that.
Second, it looks like there's some configuration work to do around choosing email addresses and even a possible authentication server, ugh.
It's not yet clear to me whether this is required or simply available.
Further investigation required!