Group visibility implemented in MDL-68093 can be set when manually creating a group.
However it would be good to have the option to set the group visibility on the import screen and/or in the CSV from the import as a new optional visibility field

Manual creation of groups (group membership visibility option available)

Import Groups (no visibility option)

Help page for Group import does not mention visibility https://docs.moodle.org/500/en/Import_groups