Joining a "group" at groups.io isn't easy; they've hidden it fairly well.
As of 2020-10-27, if you've registered an account with groups.io,
here's how I managed to join a group (aka "mailing list" in the wider internet).
- Point your browser at https://groups.io/ (of course).
- Log in if necessary, and find your groups page. It's the one that has "Your
Groups" near the top, under the blue "Groups" bar.
If groups.io recognizes you as logged in, your name will be in the upper-right corner.
- Click on the "Q Find or Create a Group" link in the blue bar. If you don't
see it, things have probably changed since I typed this, and the rest won't
work. This should get you to a "Publicly Listed Groups" page.
- Type the group name into the search widget and hit the "Q" button.
This should give a page with the group's name, and a list of buttons (with
capital letters as titles) that will take you pages for lists starting with
a particular letter. Click on your group's name (which for some reason is a
link titled "groupname / groupname", with the second in italics).
- That link will take you to the group's description page. Near the bottom,
just above the "Message History" section, is a button titled "Apply For
Membership In This Group", and click it.
- If you've registered an account with groups.io, you'll get a page saying
"Your request to join the [groupname] group has been submitted ...
- Wait for the email message to see if they've accepted you.
Supposedly you don't need a groups.io account to use it, but I don't know how
how to use it that way.