For better or worse, meetup.com has really dominated this space, but I seem to recall that craigslist used to have a section for community events and announcements where you could post something about your group. I'm not sure how many people use it though, and it had a lot of junk in it (though, to be fair, so does meetup.com).
Another way to go is the old fashioned route of posting physical paper fliers on telephone poles, or bulletin boards in cafes, universities, and libraries. It's local, absolutely free, and usually there's so much junk there that something serious and actually interesting would stand out.
I think the number of people who won't use meetup exceeds the people who won't use facebook at this point. I despise facebook but it's a necessary evil. Meetup is just a terribly clunky site I hope I never need to log into ever again. I go to some meetups that happen to be on meetup, but find the times elsewhere, or they're a weekly meetup so I already know the time. They may have dominated the space, but if you use meetup, some will just ignore the meetup group.
Interestingly, meetups are about the only thing I use Facebook now for. It’s probably the best “bbs” type site from a polish standpoint. Just ignore everything except the group pages.
* Self hosted using wordpress or (maybe) drupal. Both have event managing plugins.
* As funny as it might be, a subreddit. reddit meetups use subreddits for tracking just fine, I suppose a HN meetup could as well. The overlap between platforms is probably pretty high. In fact my recommendation would be to see if there's an existing reddit meetup group, and if any of them would be interested in a HN/tech meetup.
* Discord server with pinned messages for pending meetups.
Can anyone suggests free alternatives? I want to set one up for Waterloo, Canada but don't want to pay for a plan when I'm not sure if an audience exists.