A health service corporation shall not exclude part-time employees and shall offer the same insurance benefits to part-time employees as it offers to the employee groups of which the part-time employees would be members if they were full-time employees. The insurer shall offer to include the part-time employees as part of the employer's employee group, at the full rate to be paid by the employer, at a rate prorated between the employer and the employee, or at the employee's expense. A part-time employee shall be any employee who regularly works at least half of the weekly hours of the full-time employee in the employee group of which the part-time employee would be a member if such employee were a full-time employee, but who works a minimum of at least 15 hours per week.