Group admins can change their group settings to adjust the privacy of the group, as well as changing the group type.
How can I manage the settings of Workplace groups I've joined?
Group admin settings
What group settings are available to Workplace group admins?
How do I change the name of a group I admin on Workplace?
How do I change the privacy settings for a Workplace group I admin?
How do I change the group type on Workplace?
How do I change the default post order in my Workplace groups?