You can create a new client by clicking on the add client button on the right hand side of the toolbar on the clients tab.
The add client window asks you for some contact information for the client. If you want the client to be able to login to the Serpfox dashboard like a normal user and enjoy the full benefits of the Serpfox interface select the "Send invitation email to client" option. This will send an email inviting the user to set a password to allow them to login. This isn't essential, you may just want to configure the client account to deliver a report to the client every week rather than allowing them to login.
Once you've successfully created a client you may want to give the client access to tracking data or setup reports or notifications for the client.