Just so I'm clear on this, 'cause I have the same question:
I currently have 2 accounts: one is for a personal site, and one points to a previously-existing site that I assumed ownership of. I'm now looking to build a site for a group my kids are involved with. I'll need to buy a new domain name, I know that (or else just build it off of my own account)....if I buy a third domain name for this third site, I'll need a third account, yes? There's no way to point that third domain (for a whole new site) to one of my existing accounts, is there? (outside of just making it a subdomain?)
Thanks in advance!