You must create accounts for them under your Family Account.  Click on "Account and Coins" on the top right corner of the Club Nintendo homepage.  Then, look at the right side of the page.  It will say "Family Account Details" and will show an option to "+ADD" a Family Member.  Enter their information (which includes their emails) and then they will be able to access their own accounts and register their copies for coins.  All the coins will go into one big pot for the entire Family Account, however.

I can't seem to find a video detailing any of this, but I will give you a picture walkthrough:

Okay, so this is the Club Nintendo homepage:

You see in the red box on the top right side of the page?  It says "Account & Coins" next to your coin amount.  Click on this.

Then, scroll down until you see this box on the right:

Click on "+ADD" next to "Family Members."  The next page will tell you to input information about your family members, and then the process will be complete.