In the permissions of the specific course, you need to set it so that non-editing teachers are only allowed to view their own groups.
Under Course Administration > Users > Permissions or by clicking on the Cog icon > More > Users > Permissions (depending on your Moodle version), look for the heading "Access all groups".
Delete the non-editing teacher role from that permission (by clicking on the trashcan icon).
They should now only be able to see the members in their own group.
(You might want to remove the "Manage groups" permission as well from them, otherwise they can assign themselves to other groups as well).
https://docs.moodle.org/35/en/Capabilities/moodle/site:accessallgroups