Any eligible individual may contribute to an HSA. For an HSA established on behalf of an employee, both the employee and the employer may make contributions.
Additionally, family members may make contributions on behalf of other family members as long as the individual receiving the contribution is HSA eligible (i.e., has a qualified HDHP and no other disqualifying coverage).
For more information on how to make contributions, refer to the article How do I deposit money into my HSA?