Final answer:
The login shell is the shell program that starts after a user logs in, providing an interface to interact with the operating system. It is not involved in the authentication process nor is it common for all users in a group.
Step-by-step explanation:
The login shell is the shell program that runs when the user logs in. It is responsible for providing the user interface to interact with the operating system. After the user is authenticated through their username and password combination, the login shell becomes the main interface that the user sees and interacts with. Different users can have different login shells according to their preferences or system settings. The login shell is not specifically involved in authentication, nor is it common for all users that belong to the same group; each user can have their own unique login shell.