How can I have my client add me as an observer?

Your client can add you as an observer using the step-by-step instructions in How do I add an observer to a document set?. 

After you’ve been added as an observer, you’ll be notified via email when signature requests are sent out to the participants. The email notification will have draft copies of the documents attached. You'll be notified via email again when the document set is finalized. This email will have copies of the finalized documents attached.

Please note that being an observer alone doesn't give you the ability to make changes to a document set. If you want to have the ability to make changes to a document set, please see How can I have my client add me as a reviewer?.