If you’ve done lots of SharePoint work, you will be familiar with default groups like “All Authenticated Users” and “Everyone”.
In SharePoint Online this is different these days. There is basically just one similar group if you want to allow practically anyone on your site (usually as a Visitor). That group is called “Everyone except external users“.
The other options that were available are gone.
A few things to note about the “Everyone except external users” group:
Even though it says ‘everyone’, this does not mean that just any anonymous user can come in. This is not anonymous authentication. So in fact this is more like the ‘all authenticated users’, you do need to log in.
When you share something with a user or group in SharePoint, you get the option to send them a notification email. You also see that option for the “Everyone except external users” group, but even if you check that box, the email will not get sent.
This is by design, and it makes sense – imagine how big that group is and how many emails that would generate.