How do I become a Fan of someone's on Y!A?

You become a fan when you add someone as a contact.

Do that by hovering your mouse over their avatar and Click Add Contact

Your Fans are those that have added you as their contact.

When someone adds you as a contact they are your fan and show on your fans list and you show on their contacts list.

When you add them back or add someone you are on their fans list and they are on your contact list.

If you both add each other you both show on both lists.

When you have your privacy set to contacts only someone adding you as a fan cannot see all your q&a unless you add them back.

Some users do not allow others to add them and you will get a message that says Cannot add to your Contacts

This person is not allowing anyone to add him or her to their Contacts.