If you visit a discussion group hosted on Google Groups via web browser, you will notice that you’re asked to sign in with a Google Account or create a new Google Account when you want to join or subscribe to the group in order to receive the mailing list.
Though products of Google is popular, especially Gmail (if you have a Gmail account, you have a Google Account), but not everybody has Google Account or Gmail, nor wants to sign up for a new account just to join a discussion group or mailing list.
Anyway, if you don’t want to register a Google Account, or don’t want to use a Gmail to subscribe to a Google Group, it’s possible to join a group without logging into Google Account. Google Groups Help details the way to join or subscribe to a group via email, which does not require login to Google Accounts.
To subscribe to a group via email, send an email to [Group Name]+email@example.com.
The key is +subscribe, which is appended to the end of group’s email address. [Group Name] is the name of the group, where space is substituted by “-” (hyphen). For example, to join a group called my-digital-life, send an email to firstname.lastname@example.org.
Tip: It’s possible to find the group’s email address by visiting the group web page on Google Groups, and click on “About this group” (or “more” on top right corner and select “About” in the drop down menu. Then, look for group email address in the heading of “Group email”, where the line looks like:
Group email email@example.com
Note that you should send the subscribing email from the email address which you want to receive the messages mailing. For some groups, you may need to confirm the subscription request. In addition, for restricted-membership groups, the group owner will need to approve your subscription request before you can join.