Outlook provides a set of tools for viewing and maintaining the list of contacts in your address book. If your company uses Exchange Server, you may already use a public list of contacts from Outlook to contact someone in your organization. SharePoint provides another way to share contacts from your address book with others. Rather than providing a single, public list containing everyone's information, SharePoint is focused more on team-based or project-based lists of contacts.
For example, a Team Site might include everyone on the team in the contact list. Later, as new members join and lines of communication are established across groups, the contact list grows. In this case, the contact list is a way to share the collected knowledge of who the key people are and how to get in touch with them.
For a project site or a document workspace, the contact list obviously includes everyone with responsibilities on the project. Outside resources, such as sales people or customers, would be added as they become available.
Of course, you can also use SharePoint to share a general, company-wide list of contacts. One advantage of that approach is that SharePoint contacts are easily shared over the Internet.
Finally, there's nothing stopping you from using all these approaches to help organize contacts by company, team, and project.
It's not a great idea to add the same contact to multiple lists. If the contact's information changes, it then has to be changed in all the lists. Instead, it's a good idea to follow rules about where you store contacts and how you use them. Here are some suggestions:
Decide whether you are going to use SharePoint or Exchange Server to share company-wide contacts.
If using SharePoint for company-wide contacts, organize those contacts into one or more lists at the top-level site.
Restrict who can add or change contacts in the top-level SharePoint lists.
Use project or workspace contact lists as temporary resources that have a limited lifetime.
A company might provide Employee, Customer, and Vendor contact lists in its top-level site that can't be edited by most members, but then allow team members to create their own ad hoc contact lists in team sites and workspaces. Although the ad hoc lists might become out-of-date, they allow members to organize the contacts that the team needs and perhaps include contacts that don't belong in the company-wide lists.