Learnability
The overall design is pretty straightforward. Information is added within each section and is immediately updated after it is added. All the buttons have the typical affordances for clicking, and there is a consistent design on what an add button looks like. Edit links will show up next to items that can be changed. For tasks and documents, underlined items are clickable as usual. Deleting items is possible after getting to the edit menus. The website doesn't create any new widgets or items not already commonly found on the web, and an experienced web user should be able to navigate the site quite easily.
Visibility
The side panel allows for visibility of how information is treated within the website: companies are within groups, and tasks, documents, and contacts are within each company. The design allows for quick access to all the tasks, contacts, and documents through links visibly placed in the navigation bar. However, it isn't clear from the design that the links in the navigation bar show all the contacts, tasks, and documents the user has uploaded. The function of these links will become apparent after the user clicks on the first one.
Efficiency
The design is pretty efficient at looking up already stored information, since there are various methods to find an item (i.e. contact, task, document). One can select the company the item is affiliated with, look at the appropriate subgroup on the Companies page, or click the appropriate navigation bar item. Each item in the navigation bar shows the user all the contacts/documents/tasks she has uploaded, and these can be filtered for easier lookup. The design does not provide an efficient way to upload batch information, however. If the user has 100 companies each with contacts, tasks and documents there isn't an easy way to add all the items at once from this design. However, users are unlikely to need to add so much information at once.
Error Prevention
When adding companies, existing groups are visible in the drop down menu, so a user won't accidentally add redundant groups. To prevent duplication, a dialog box will indicate an error when a user accidentally tries to add a company twice. Errors in adding the information are easily correctable, however deleting things can't be undone. It will be apparent to the user when and what he/she is deleting when that happens, so it's highly unlikely there would be an error.