Most multi-user Wordpress blogs use user roles such as "Contributor" which allows users to write and send posts for moderation. But one thing which lacks in this role is ability to upload files. So, here is a simple code to enable it.
Open functions.php file of your theme and paste the following code:
if ( current_user_can('contributor') && !current_user_can('upload_files') )
add_action('admin_init', 'allow_contributor_uploads');
function allow_contributor_uploads() {
$contributor = get_role('contributor');
$contributor->add_cap('upload_files');
}
It will enable upload files feature for contributors.
This code was shared by Ryan Marganti