There are two ways to create a free attendee account. An attendee account establishes a permanent identity, making it much easier to manage your action items, meetings and notifications.
The first way is to go to http://instantagenda.com/ and click "Sign in". There, you will be given the option to sign in via Email or Google account. Choose one or the other, and it will then prompt you to create an account (this is automatic for Google Accounts).
The second way is available whenever you join a meeting. When you attempt to join a meeting, you are prompted to join as a Guest. You can also sign in or create an account (you can see this highlighted in the image below).