Often guest authors will post one time only. You don’t want to set up an account for them so you post under your own name. You can use this snippet along with custom fields to change the post author name to the guest author. Add this to your functions:

add_filter( 'the_author', 'guest_author_name' ); 
add_filter( 'get_the_author_display_name', 'guest_author_name' ); 
function guest_author_name( $name ) { 
global $post;
$author = get_post_meta( $post->ID, 'guest-author', true ); 
if ( $author ) 
$name = $author; 
return $name; 

Then add a custom field called guest-author and put the author’s name into the value.