Short answer: Usually not. Group plans cover spouses, dependent children, and; where offered; domestic partners who meet the plan’s definition. A boyfriend or girlfriend generally doesn’t qualify unless the plan specifically offers domestic-partner coverage and the couple meets its criteria.
Standard group coverage extends to a legal spouse and dependent children, and some plans add domestic partners. A simply cohabiting boyfriend or girlfriend typically isn’t eligible unless the employer’s plan offers domestic-partner coverage and the couple satisfies its requirements (which often include shared residence and finances, and an affidavit). Even where allowed, the value of the partner’s coverage may be taxable to the employee unless the partner qualifies as a tax dependent. Check the plan’s eligibility definition.