See also the official documentation:
Help Center > Members Manual > Logging in to your account

Logging In Without a Password

Groups.io makes it simple and free to log in — you don't even need a password:

1. Use this link to go to groups.io using your browser on ANY internet enabled device:
   https://groups.io/sendloginlink
2. Enter your email address — be sure to enter the address where you receive this group's messages.
   If you are not sure, look in the footer at the very bottom of any message you've received by email.
3. Click the green Email me a link to log in button:
   
   Monitor your email because a special link will be sent to the email address you entered.
4. When you get the email on your device, Click on the link or copy and paste it into your Internet browser
   You should  be taken to a web page where you can see a link to this group.

The first thing you should do is to Bookmark that page (https://groups.io/) in your Internet Browser so you can find your groups again.

If you don't receive the Log In email within a few minutes, double-check that you've entered your email address correctly. Also check your email Spam or Junk folders in case your email service didn't deliver the message to your Inbox.
The link in the message will expire in 24 hours; if that happens and you can't log in, start at step 1 again.

Your email address is your account ID. These steps log you in to Groups.io, and to any groups that you subscribe to using the same address. You will stay logged in for 30 days, extended each time you visit.

If your login expires you can repeat this process any time you wish. You may also repeat this process on other computers, browsers, or devices you wish to use, you do not need to log out of one to use another.

Some devices automatically delete cookies, so you'd need to log in every time.

See this page if you want to set an account password or need more detailed information.