Currently, only a Performer can add/edit agencies within their account. Your client can add you as an agency in their 'Manage my agents' section. Once that's done, you're all set!
However, if you want us to add or remove a client for you, please click the 'Submit a request' link at the top of the page, and choose the options below when prompted.
We hope to release this feature to agencies to add their own clients in the near future.