If an employee or his/her dependents have a qualifying event making them eligible for special enrollment on the group health plan, when is the effective date of the coverage?

Federal regulations set the standards for the effective date of coverage if enrollment is due to a qualifying event.

Carriers can (and some do) make coverage effective sooner than required by the federal standards.

From the Department of Labor’s FAQ document on HIPAA special enrollment rights: https://www.dol.gov/sites/default/files/ebsa/about-ebsa/our-activities/resource-center/faqs/hipaa-compliance.pdf