Section 2(6) defines aggregate Turnover which includes exempt supply. Exempt supply - 2(47) in my understanding excludes supplies set out in Sch II. Schedule III (1) considers supply b/n employees to the employer not a supply of either service or goods. Section 24 ((1) (vii) specifies that those who make taxable supplies on behalf of the taxable persons are liable to be mandatorily regd.
In this case the employee who provides some marketing services and earns a commission cannot be said to be providing the supply on behalf of another taxable person. he is providing services to that person.
Therefore if salary income is 24 L and commission income is less than 20 lakhs, there is no need of registration unless he is liable to pay GST under section 9 (3) under Reverse charge mechanism.