If you work on your own, the 'about us' section will mainly be biographical information on you, demonstrating the experience that makes you a good agent and mentioning any high profile sales you have made. If you run a larger agency, you can also include brief bios of the other agents you work with. In either case, you should mention when the agency started - the longer it's been running, the more creditable it will look.
Promoting your authors
Your list of authors doesn't only publicise them. It also shows the range and quality of your agency. As space isn't a problem on the web, it's sensible to offer more than just a list of names. You could mention their best known books, include any other claims to fame such as TV appearances and, in the case of non-fiction writers, describe their areas of expertise, It's also a good idea to provide a link to their websites and encourage them to link back to you. (The more inward links you have, the higher your chances of good search engine rankings.)
If you decide to include information on rights, you can either list all the books you represent or pick out a selection that you feel are particularly marketable. If you decide to state exactly which rights are still available, make sure you keep this update.
It's sensible to include these, even if you're not looking for new clients. Providing clear details of what you do and don't want to see may help stem the tide of unsuitable submissions (although it's unlikely to stop it completely),