Ref: https://support.google.com/groups/answer/46601
There are two versions of Groups
the free version for everyone, and
a version for organisations (work, school, and so on)
aka Google Groups for Business.
The tasks you can do and features you can access depend on which version you have. If you're signed in with a work or school account, the tasks and features available to you are customized by your administrator.
If the group address has the format https://groups.google.com/g/groupname , then it is free version Google Group.
If the group address has the format https://groups.google.com/a/domainname/g/groupname , then it is a business version.
If you see the 'content unavailable' message...
This means you are either not signed in to any Google account or are signed in to a Google account that does not have access to the group you are trying to reach. See the sections below about Google accounts and how to tell which account you are using.
A common scenario is when an invitation email from the group is forwarded to a personal gmail account or to 3rd party mail app where the default browser opens defaulted to a personal gmail account, as opposed to the Google account for the group. Then the link to the Google Group tries to open in a browser with the "wrong" Google account logged in.
To use Google Groups (and many other Google products), users must have a Google account:
This can either be:
Google email address account (typically ...@gmail.com but can be any Google Workspace domain address)
or
A Google account associated with any existing (non-Google) email address (e.g. hotmail, yahoo, bt.com etc)
In the second case, if they haven't created a Google account using their existing email address, they will need to do that first.
They can go to https://accounts.google.com/signin and select > Create account > For my personal use > Use my current email address instead.
Sometimes users looking at https://groups.google.com/my-groups don't realise they are signed in to Google with a different account to the one they think they are signed in with. To check which account they are signed in to, they should hover their mouse over the icon at the top right of the My Groups screen. The account they are signed in with will appear as indicated by the red rectangle.