Office 365 Groups and Team Membership
It is possible to get into an orphaned Team state where there are no "Owners" within Teams, but adding a member to O365 group does not add that as the owner within Teams.
This makes it possible to lock yourself out of Teams if the last owner leaves the team, it is not possible to add another member.
We tested this on May 19th/20th. Removing a member from the O365 did not remove them from Teams, and an Owner on the O365 group could not see the team unless they were added from within Teams.
When adding a member to an Office 365 group, it should give you access to all services tied to that O365 team. This can make it difficult to segment out access for users who just need Teams access without all the same access as the O365 group provides.
I recommend that when you add a user you select their role and then toggle which services they have access to, much like the licensing options under the O365 portal.
This helps administrators and users understand what services are being provided to each user. This makes it easier to decommission unused O365 groups without worrying about losing data within those groups.