If you create an Office 365 group, it is now private by default. Only the approved members of your organization can be a part of that group and see that group’s content. People who want to join a private group have to be approved by a group owner.
Can a group be changed from private to public?
Yes, it can but only by group owners. They can add or remove members, as well as make changes to the group such as renaming, deleting conversations, and more.
Can we provide group access to people outside our organization?
Yes, by granting them access as a guest. For more information, see Guest access in Office 365 groups.