From FAQ about Multiple Users (for groups)

How do I add more authors / users to my blog? [screencast]

If you’d like your blog to feature posts from multiple authors, you can add as many administrative accounts to your blog as you like. There are two different ways to add a new user to your blog, depending on whether or not that person already has an account at WordPress.com.

To add someone who already has a WordPress.com username:

1. Log in to your dashboard
2. Click on the Users tab
3. In the Add User From Community section at the bottom, enter the author’s email address in the User E-mail box
4. Select either Contributor, Author or Editor in the Role list
5. Click the Add User button

Your new contributor will now see your blog listed in their dashboard when the log in to WordPress.com.

To add someone who does not have a WordPress.com username or blog:

1. Log in to your dashboard
2. Click on the Users / Invites tab
3. Enter the author’s first name, last name and email address in the appropriate fields
4. Check the box labelled Add user to my blog as a contributor
5. Click the Send Invite button

Your new contributor will receive an email inviting them to sign up for a WordPress.com account (and, optionally, create their own blog if they want). Once they have signed up, you’ll see them listed as a Contributor on your Users tab. You can change them to a different role (Author or Editor) later if you’d like.

source link : http://faq.wordpress.com/2006/05/17/how-do-i-add-more-authors-users-to-my-blog/