Almost all plans allow you to add a legally married spouse.

Biological, adopted, step, and foster children can stay on your plan until age 26, regardless of their marital or financial status.

If the person doesn't qualify as your dependent (e.g., an adult child over 26 or a friend), you can still help them get covered through the Health Insurance Marketplace: Who's included in your household | HealthCare.gov

Yes, you can buy health insurance for someone else, but the method depends on your legal and financial relationship with them. You can either as a dependent or pay for a separate plan in their name. 1. Adding Someone to Your Own Plan

Many employer-sponsored and some state-specific plans allow unmarried partners if you provide proof of cohabitation or shared finances.

You can typically add individuals to your existing policy if they meet specific eligibility criteria as a :

Generally, friends, roommates, or parents cannot be added unless you claim them as a tax dependent , have legal guardianship, or provide more than half of their financial support. 2. Buying a Separate Marketplace Plan