In the Microsoft Office 365 suite, University students and staff can create Groups. When creating a Group, the default visibility is set to public, but you will have the option to make this a private group at this point.
Please be aware that content in a public Group can be seen by anybody in the University and anybody in the University can join the Group. Content in a private group can only be seen by members of the group and people who want to join have to be approved by the Group owner/admin.
If you do wish to change from public to private (or vice versa), a guide of how to do this can be found at https://support.office.com/en-gb/article/Make-Office-365-groups-public-or-private-c0a991b3-9c56-48b8-bf0f-05530f836b1b