See here for adding users http://en.support.wordpress.com/adding-users/ and note that when you do add users there will be author bylines, provided the theme is designed to display them.
Some themes display author bylines and some do not. You can switch to a theme that does display a byline. http://wpbtips.wordpress.com/2009/08/15/author-displayed-or-not/
Many other themes on WordPress.com do not display the author name on blogs with only one author. The point of displaying an author name is to differentiate the posts on an multi-author site.
There is, however, a workaround that single author bloggers can use to compel the display.
1. While logged out of WordPress.com create another username account with a different email address. next add the new username as an Author. Log-out and clear your browser cache and cookies.
2. Now log-in as the new author ie. username account and publish one post on the blog but and backdate it to the date you started the blog. (example here http://thistimethisspace.com/2007/04/24/maintenance-post/)
I know you are not scheduling a post but to backdate the datestamp see the images here and you will know what to click to edit the date. > http://en.support.wordpress.com/posts/schedule-a-post/
Log-out and clear your browser cache and cookies.
3. Lastly, log in under the original username account and you will have an Author byline the next time you publish a post.
This may also be useful > http://onecoolsitebloggingtips.com/2011/03/06/multi-author-wordpress-com-blogs/