Whether or not you have many users on your system, it's important to understand the aspects of user management under Linux. Even if you're the only user, you should presumably have a separate account for yourself (an account other than root to do most of your work).
Each person using the system should have his or her own account. It is seldom a good idea to have several people share the same account. Not only is security an issue, but accounts are used to uniquely identify users to the system. You need to be able to keep track of who is doing what.