There seems to be a discrepancy regarding how/when the "Invitation"/"Welcome" emails sent to guest users that are added to different types of O365 Groups.
If an Outlook Group is created (either in OL2016 or the O365 Portal) and guests are added, it appears that the invitation email is being sent. In many cases (*cough* Gmail *cough*), that message is being thrown into the Spam folder, but it is there. When people click the "Access files" link, they can get to the underlying SharePoint site.
Now, let's say you create a Microsoft Team. The Teams provisioning engine creates an Outlook Group of the same name and then the other components like the SharePoint site. If you add a guest user to the Team, they get an invitation to use Microsoft Teams, but that invitation doesn't include a direct link like the "Access files" link in the Outlook Group invitation. So, you remove them from the Team and go and add them to the Outlook Group (so that they can get an invite with the Access Files link), but the invite isn't sent.
In other words, the invitation is only sent when the guest user is added in the same endpoint where the O365 Group is created. Maybe this is by design, but if so, it's not a good design.
The problem is that "guest users" are handled a bit differently between Teams and Groups, it's not just the email bit. And yeah, it's annoying, but that's what happens when you have to at least try to keep up with all the promises the marketing team is making :D