In case you missed the announcement, WordPress.com blogs can now support more users than just you, the main user.
From the Users panel in the Adminstration Panels, the second sub-panel is called Users and Authors. Part way down the page is the form to add users to your WordPress.com blog.
Enter in the user name from the person’s WordPress.com account. Currently, only registered WordPress.com bloggers are permitted to be added to your WordPress.com blog. In the next box, choose their level of access. You can learn more about the specifics of access through WordPress Codex – Roles and Capabilities of User and Authors.
Now, you can have guest bloggers or more than one blogger on your WordPress.com blog! Have fun!