Add new users so each person has their own login. This way when they post to the blog they will have their name listed as the author by default. You can also limit user ability by choosing the level you assign to each user.
Under users click add new
Fill in the fields and select a user level
Summary of roles:
i. Administrator – Somebody who has access to all the administration features
ii. Editor – Somebody who can publish posts, manage posts as well as manage other people’s posts, etc.
iii. Author – Somebody who can publish and manage their own posts
iv. Contributor – Somebody who can write and manage their posts but not publish posts
v. Subscriber – Somebody who can read comments/comment/receive news letters, etc.