This would certainly be possible. If you wanted someone to just have access to your invoices within the client area, or perhaps access to domain management, you would just need to create a separate login via your client area which they can use.
You can do this by selecting: My Details > Add New Contact
When adding a new user, you just need to fill out their contact details. If they need access to certain parts of your account, you'll just need to select the option to 'Activate Sub-Account'. Once activated, you can then select which privileges you would like them to have (which parts of your account you would like them to have access to).