Based on your post, I understand that you want to add all users of a tenant into a SharePoint group. Is it right?
Here are steps:
1.Enable "Everyone" group by using PowerShell.
#SharePoint Online Admin Center URL
$AdminURL = "https://tenant-admin.sharepoint.com/"
#Connect to SharePoint Online
Connect-SPOService -url $AdminURL
#Disable Everyone group
Set-SPOTenant -ShowEveryoneClaim $True
2.Go to the site collection -> Site permissions -> Advanced permissions settings -> Click the sote members group -> New -> Add Users -> Enter Everyone group.
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