External access to O365 Groups access from the external user point of view

The "standard" email when you are invited for an Office 365 Group is different for external users. In the external user's email, there are no links to check out the calendar or take part in conversations. That is because, what Vasil already said, external users don't have access to the Exchange part (conversations and calendar) of a Group. Nor do they have access to Planner, OneNote, Teams, Stream, etc. for that matter. The only place external users have access to, is the SharePoint site that is connected to the Group. So the person who invited the user, can give the URL to the SharePoint site.

You'll see that for internal users, when they visit the SharePoint site, there is a link to go to the conversations of that Group in the right top corner. When external users access that same site, the link is not there. Again, because they don't have access to the conversations. They can still participate in conversations, but through email only.