I Have A Second Family Member Who Needs To Register.

The previous answer is not really correct:

1. The "second family member" must be, at the same address, a spouse or partner. A brother, for example, does not qualify.

2. If you've already identified this person on your membership application, you're done--but as you're asking the question, this must not be the case. If you had identified him or her in that manner, he or she would have received a separate membership card.

3. You can identify this person now (that's probably what you mean by "register"). Just inform AARP: Use the contact information here--https://www.contacthelp.com/aarp/customer-service. Expect a separate membership card.

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If their membership fee is paid there's no reason to register. It is done automatically.