Create a company and user in one, what's the standard?

I know a lot of startups and tech companies essentially allow you to register, and you end up registering a company and your user.

An example would be basecamp for example. I'd like to achieve the same thing, however I'm not quite certain on how they do it, and what the best way to do it is.

My thought is to have a user and company model, where on registration you register a company, and it accepts nested attributes for user. As in my head at least the relation is:

 User belongs_to :company
 Company has_many :users

and the registration is a Company#new with a company.user.build.

However for some reason this does feel a bit strange, as to me it would make more sense that you register a user, and create the company it belongs to.

I just want to lay the foundation right, so I don't start building anything massive on top of a system that isn't good.