All posts and pages have one or more authors. The default author is the user who created the post, but more authors can be added and the original author can be removed given that there is at least one more author assigned to that post. In fact, authors do not even have to be users -- profiles may be created for "Guest Authors" and then these profiles may be assigned as authors on posts. Note that this post has two authors, as is displayed in its byline.
Assigning Authors to Posts
To be able to add or change post authors, the Authors section must be enabled on the post's edit screen. This can be done by checking the Authors box under Screen Options at the top of the page.
To add an author, start typing their name in the box labeled 'Search for an author'. The author should then appear below the search box, and you can click on the result to add the author to the post.
Once a post contains multiple authors, you may click and drag an author to change the order they appear in the byline. A Remove link also appears next to each author's name. If you click on the name of an author you can edit their display name.
Adding a Guest Author
To add a guest author, go to Users > Guest Authors and click Add New.
On the Add New Guest Author screen, fill out any applicable fields and click Add New Guest Author under Save. You may now search for and add this author in a post just as you would any user.