We cannot edit templeates and themes on free hosted wordpress.com blogs. Not all themes are designed to provide an author profile and byline. Only a few display an author profile below each post; most themes do not. In themes that are coded to provide the aothor profile below each post that dat is drawn from here > Users > My Profile > About you
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.
For a single author byline workaround, and for a link to a list of themes that do display author profiles see here > http://onecoolsitebloggingtips.com/2012/01/03/wordpress-com-changes-bylines-for-authors/
As you are creating a multiauthor blog this may be helpful > http://onecoolsitebloggingtips.com/2011/03/06/multi-author-wordpress-com-blogs/