Navigate to a project, click "show client details" and click add contact. You can add multiple contacts to a single project but each contact has to be associated with a single client organization. Each client contact will receive project notifications and any project surveys you have setup for clients.
To get started, click Show Client Details above the project tabs.
This displays any client contacts associated with your project. You can add as many contacts as you like, start by clicking Add Contact button.
This will open up a list of all existing clients for the project sponsor organization. It may be empty, but you can select New Contact to add a new person to your project who did not already exist in your contact database.
Repeat this as often as needed to complete your list of client contacts on a project.