Unable to add users to Farm Administrator's Group SharePoint 2013

In SharePoint 2010, you got a nice error message when you try to add a user to the Farm Administrators group in Central Administration.

In SharePoint 2013, not so much:

This happens even if you are a member of the Farm Administrators group.
If you look up the event in the ULS viewer, you see the answer (which is the same for SharePoint 2010 as well).

Local administrator privilege is required to update the Farm Administrators' group.

So add your account or use an account that is a member of the local admin group on your SharePoint front-end and you should be good to go. 

Best practice is to just use a local admin account to add the user and NOT add yourself to the local administrators group.

Also check out: 

You cannot add users and groups to the Farm Administrators group after you remove the BUILTIN\Administrators group from the Farm Administrators group in Windows SharePoint Services 3.0
http://support.microsoft.com/kb/926320