Regardless of the theme, bylines will only display now if there are at least two authors who both have at least one published post in the blog. This post announced several theme enhancements from November 2011 http://en.blog.wordpress.com/2011/11/21/theme-improvements/
Only a few themes also have an additional author profile that appears at the end of posts. Currently Chateau, Choco, Duster, Elegant Grunge, Enterprise, Liquorice, Matala, Mystique, The Morning After, Next Saturday, Pink Touch 2, Selecta, Skeptical and Twenty Eleven, Twenty Ten, Vertigo and Vostok display the author’s profile and gravatar when viewing a single post at the end of the post.http://wpbtips.wordpress.com/2009/08/15/author-displayed-or-not/
Read also: Multi-Author WordPress.com Blogs
If the authors on your blog have been added as official users to the blog and all have published a single post on the blog then their bylines ought to be appearing. If they aren't then Theme Staff will reply to this thread and make the required fix. Please be patient while waiting for Theme Staff to respond to this thread.