When you're using your personal profile you can follow or add any person or business to your circles and they get a notice and have the option to circle you back. You can +1, share or comment on anything using your personal profile name.
When you're using your business page you can follow or add other businesses to your circles, +1, share and comment on their posts using your business name. They get a notice and can follow you back if they want to. You can't circle or follow a person or comment on their posts using your business name unless they circle you first but you can +1 their posts and share them. When you try to add a person that hasn’t added you first to your business circles it doesn't give you an error but it doesn't add them either and their little card just floats back to the top when you try to drag it into a circle. You can tell that it didn't work because the number of people in your circle doesn't increase. You can always remove people from your circles if you decide that you don’t want to see their posts after all.