I've just gone into my account and signalled that I want to add on a domain. It's not clear whom to contact, or how to actually purchase the domain name. Can you advise on how to proceed?
You need to purchase the domain from a registrar, like GoDaddy or Gandi.net. Then email us with the details and we will connect it up for you.
There are some details here on the process:http://nationbuilder.com/can_i_use_my_own_domain_name