The first thing you have to do is sign up and create an account. An account might be the entire company, or it might be one department in a larger organization. (When Outages is fully released, the account is what will get billed.)
When you sign up with the Sign Up link, you become to owner of a new account. You will be taken to a page where you enter the account information. Note: that means you don't want to sign up this way if you just want to join an existing account. You have to be invited to join an existing account.
Once you're created the account, you can invite other people in your organization by going to the Users page and clicking the "Add User" button, or by going straight to the Add User page.