One of the most appealing aspects of Google's applications is that in most cases they're accessible to anyone who wants to give them a try. If they're not, it's usually because the application is so new that the powers that be over at Google are phasing in the number of users.
Beginning with Gmail, Google has allowed a limited number of users to sign up for applications while they are still in beta testing. Once the limited number is reached, new users are allowed to access an application only by invitation until the beta testing on the application is complete. At that time, Google opens the application to general use.
Whether you're one of the first users to access a new application, a user who's fortunately enough to land an invitation (because you know "in-people"), or someone coming to a program after the application has been opened to the general public, the first step is to create a Google account.