O365 Group members that are external guests that don't yet have Microsoft Accounts aren't prompted to create one on the site's logon page.
The logon page when you visit an O365 group doesn't have the "No account? Create one!" link to allow someone who was added as a member of an O365 Group to create a Microsoft Account anymore. This directly affects External Guests who don't have Microsoft Accounts. There is no visible link for them to create one. They don't know what to do as it's just prompting them to logon.