This is pretty complex. To answer the title question, I think is out of the Moodle logic to use groups inside a course to restrict the access to the upper level. You could put all the course content in one section and use the restriction settings -> group on the section.
However If I understood the whole thing I think you should use the enrolment duration feature, instead. So you enrol each employee to the course he want to partitipate in, setting the enrolment start and end.
However If I understood the whole thing I think you should use the enrolment duration feature, instead. So you enrol each employee to the course he want to partitipate in, setting the enrolment start and end.