Each client needs to sign up individually. If you're an agency, you can use the same email address and contact information for each account, but you will need to register each account separately. We will also need the account information (Google account, Yahoo username & password, MSN username & password)
We're working on a solution that will allow agencies to see multiple accounts under a single login, but it hasn't launched yet. Even then, you'll have to provide each client's information individually.