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Once the user is logged into the site, they are offered several different pieces of information, each of which take up their own corner of the screen real estate. In the top left, there is the total amount that the user owes, and the number of overdue bills appears in a large, red font. This will quickly attract the user's attention to any overdue bills, which we expect them to correct promptly. On the right side of the screen, the user sees their bills for the current month. This calendar serves as an overview. There is a larger version of this calendar available on a different tab, if a user wants to explore further than the bills for the current month. Next, there are two very prominent buttons towards the bottom of the screen, allowing the user to add a bill or to pay a bill. We expect these to be very common tasks on our site, and as such made these actions stand out on the page. The buttons are large, which comes from the application of Fitt's Law.
One of the main tasks that a user will perform on the site is adding new bills. Because of this, we try to make this action straightforward, helping the user along as much as possible. For instance, when a user inputs the total amount for a bill, that amount is automatically subdivided up amongst all of the selected users. Similarly, a calendar pops up when the user wants to select the due date for the bill. This makes the selection process a simple point-and-click, instead of using drop-down menus or another selection scheme.
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A user can see all of their bills on a single page. We took an idea from the way that groupon lets users organize their coupons. In particular, users can see bills that have already been paid, and those that are yet to be paid. This gives us a high degree of error correction, in case a user accidentally marks a bill as paid. This means that bills are never actually deleted, and gives the user a full record of their payments. The bills are arranged in a table, ordered by date due. 
Any bill that has not been paid can be edited by the user. This page looks very similar to the page for adding a bill, except for the fact the the existing bill's information is pre-loaded into the text box fields. In this way, the user can exactly see the bill state at any time, and can change any aspect of the bill.
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