Currently, updates and beta are the only public groups of any note. Since I would like to see this platform take off, I would like to make a suggestion about how this could occur.
Instead of the default tab of groups being "Most popular", I would suggest temporarily making the default a new "Featured" tab instead. Featured would include the updates and beta groups, but also some generic groups that you create yourself, such as "Linux", "Apple", "Startups", "Programming", or "Web Development". By making these the most prominent displayed groups, people would be more likely to join them. The groups should be chosen to be the groups that members are most likely to join. If any public groups start to succeed organically, then they could be added to featured as well. Regardless, having the fourth group in the list being "Poop" isn't a good look. I would also suggest promoting this set of initial seed groups via your mailing list.
At the start, you should be able to handle moderation of these groups yourself, but once they take off you should be able to pass moderation onto the community members. Hopefully this would be enough to get the Groups.io community started.
I would be willing to join these groups and make a few posts so that they at least have some content.
1. Mightn't this make the site unwelcoming to those who aren't programmers?
I imagine there would other topics for which you could create default groups, but I could only think of programming one's off the top of my head. Also, the nice aspect of the featured tab is that it enables you to promote groups in order to expand the audience of the site to different kinds of people.
2. Wouldn't it be better for groups to form organically?
As soon as some groups do form organically, then they could be moved on top of the featured tab or the featured tab could be removed. Furthermore, the ability to create sub-communities mitigates many of the issues. If a "Programming" community is too broad, then they can change their focus to general programming questions and shift questions on specific languages to sub-communities such as Java, C++ or PHP.
Anyway, I just thought I'd throw this suggestion out there because I'd love to see this product gain more traction.