Multiple roles per user WordPress plugin. Working when creating a new user and editing an user. Thanks to nikolov-tmw (https://goo.gl/Xv563d) and evankennedy (https://goo.gl/sh2sX6).

multiple-roles-per-user.php

/**
 * Plugin Name: Multiple Roles per User
 * Description: Allows anyone who can edit users to set multiple roles per user. In a default WordPress environment that wouldn't have much of an effect, since the user would have the privileges of the top-privileged role that you assign to him. But if you have custom roles with custom privileges, this might be helpful.
 * Version: 1
 * Author: nikolov.tmw
 * Author URI: http://paiyakdev.com/
 * License: GPL2
 */
/*
	Copyright 2013 Nikola Nikolov (email: nikolov.tmw@gmail.com)

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2, as 
    published by the Free Software Foundation.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

function mrpu_plugin_init() {
	load_plugin_textdomain( 'multiple-roles-per-user', false, dirname( plugin_basename( __FILE__ ) ) . '/languages/' ); 
}
add_action( 'plugins_loaded', 'mrpu_plugin_init' );

function mrpu_admin_enqueue_styles( $handle ) {
	if ( 'user-edit.php' == $handle || 'user-new.php' == $handle ) {
		wp_enqueue_style( 'mrpu-style', get_template_directory_uri() . '/css/custom-style.css' );
		$mrpu_inline_style = 'label[for="role"], select#role { display: none; }';
		wp_add_inline_style( 'mrpu-style', $mrpu_inline_style );
	}
}
add_action( 'admin_enqueue_scripts', 'mrpu_admin_enqueue_styles', 10 );

/**
 * Adds the GUI for selecting multiple roles per user
 */
function mrpu_add_multiple_roles_ui( $user ) {
	$roles = get_editable_roles();
	$user_roles = array();
	if( $user == 'add-new-user' ) {
		if( !current_user_can( 'create_users' ) ) return;
		$user_roles = in_array( $_POST['mrpu_user_roles'], array_keys( $roles ) );
	} else {
		if( !current_user_can( 'edit_user', $user->ID ) ) return;
		$user_roles = array_intersect( array_values( $user->roles ), array_keys( $roles ) );
	}
	?>
	<div id="mrpu-roles-container">
		<h3><?php _e( 'User Roles', 'multiple-roles-per-user' ); ?></h3>
		<table class="form-table">
			<tr id="mrpu-roles-container-tr">
				<th><label for="user_credits"><?php _e( 'Roles', 'multiple-roles-per-user' ); ?></label></th>
				<td>
					<?php foreach ( $roles as $role_id => $role_data ) : ?>
						<label for="user_role_<?php echo esc_attr( $role_id ); ?>">
							<input type="checkbox" id="user_role_<?php echo esc_attr( $role_id ); ?>" value="<?php echo esc_attr( $role_id ); ?>" name="mrpu_user_roles[]"<?php global $pagenow; if (( $pagenow == 'user-edit.php' ))  { echo in_array( $role_id, $user_roles ) ? ' checked="checked"' : ''; } ?> style="width: 16px;" />
                            
                            
                  
                            
							<?php echo $role_data['name']; ?>
						</label>
						<br />
					<?php endforeach; ?>
					<br />
					<span class="description"><?php _e( 'Select one or more roles for this user.', 'multiple-roles-per-user' ); ?></span>
					<?php wp_nonce_field( 'mrpu_set_roles', '_mrpu_roles_nonce' ); ?>
				</td>
			</tr>
		</table>
	</div>
	<script type="text/javascript">
		(function(callback) {
			var prefix = '', done = 0,
				action = addEventListener||(prefix='on',attachEvent),
				unaction = removeEventListener||detachEvent;
			var caller = function() {
				if(!/complete|loaded|interactive/.test(document.readyState) || done++) return;
				callback();
				unaction.call(window, prefix+'load', caller, false);
				unaction.call(document, prefix+'readystatechange', caller, false);
			};
			action.call(window, prefix+'load', caller, false);
			action.call(document, prefix+'readystatechange', caller, false);
			caller();
		})(function() {
			var elem = document.getElementById('role');
			var mrpu = document.getElementById('mrpu-roles-container');
			var mrputr = document.getElementById('mrpu-roles-container-tr');
			while(elem && elem.tagName != 'TR') elem = elem.parentNode;
			if(elem && mrpu && mrputr) {
				elem.innerHTML = mrputr.innerHTML;
				mrpu.parentNode.removeChild(mrpu);
			}
		});
	</script>
<?php }
add_action( 'edit_user_profile', 'mrpu_add_multiple_roles_ui', 0 );
add_action( 'user_new_form', 'mrpu_add_multiple_roles_ui', 0 );

/**
 * Saves the selected roles for the user
 */
function mrpu_save_multiple_user_roles( $user_id ) {
	// Not allowed to edit user - bail
	if ( ! current_user_can( 'edit_user', $user_id ) || ! wp_verify_nonce( $_POST['_mrpu_roles_nonce'], 'mrpu_set_roles' ) ) {
		return;
	}
	
	$user = new WP_User( $user_id );
	$roles = get_editable_roles();
	$new_roles = isset( $_POST['mrpu_user_roles'] ) ? (array) $_POST['mrpu_user_roles'] : array();
	// Get rid of any bogus roles
	$new_roles = array_intersect( $new_roles, array_keys( $roles ) );
	$roles_to_remove = array();
	$user_roles = array_intersect( array_values( $user->roles ), array_keys( $roles ) );
	if ( ! $new_roles ) {
		// If there are no roles, delete all of the user's roles
		$roles_to_remove = $user_roles;
	} else {
		$roles_to_remove = array_diff( $user_roles, $new_roles );
	}

	foreach ( $roles_to_remove as $_role ) {
		$user->remove_role( $_role );
	}

	if ( $new_roles ) {
		// Make sure that we don't call $user->add_role() any more than it's necessary
		$_new_roles = array_diff( $new_roles, array_intersect( array_values( $user->roles ), array_keys( $roles ) ) );
		foreach ( $_new_roles as $_role ) {
			$user->add_role( $_role );
		}
	}
}
add_action( 'edit_user_profile_update', 'mrpu_save_multiple_user_roles' );
add_action( 'user_register', 'mrpu_save_multiple_user_roles' );

/**
 * Gets rid of the "Role" column and adds-in the "Roles" column
 */
function mrpu_add_roles_column( $columns ) {
	$old_posts = isset( $columns['posts'] ) ? $columns['posts'] : false;
	unset( $columns['role'], $columns['posts'] );
	$columns['mrpu_roles'] = __( 'Roles', 'multiple-roles-per-user' );
	if ( $old_posts ) {
		$columns['posts'] = $old_posts;
	}

	return $columns;
}
add_filter( 'manage_users_columns', 'mrpu_add_roles_column' );

/**
 * Displays the roles for a user
 */
function mrpu_display_user_roles( $value, $column_name, $user_id ) {
	static $roles;
	if ( ! isset( $roles ) ) {
		$roles = get_editable_roles();
	}
	if ( 'mrpu_roles' == $column_name ) {
		$user = new WP_User( $user_id );
		$user_roles = array();
		$_user_roles = array_intersect( array_values( $user->roles ), array_keys( $roles ) );
		foreach ( $_user_roles as $role_id ) {
			$user_roles[] = $roles[ $role_id ]['name'];
		}

		return implode( ', ', $user_roles );
	}

	return $value;
}
add_filter( 'manage_users_custom_column', 'mrpu_display_user_roles', 10, 3 );

There is a little bug in this plugin when changing roles assigned to a user, in the user edit screen.
Cheers

This is brilliant thanks!!
What's the bug? Seems to work fine for me.

Hi @jtibbles!
Thanks! Well, now I found the problem: in my case, I use User Role Editor plugin.
When User Role Editor is active, and I try to change roles assigned to an user, in the user edit screen, I can only unassign one role per time, a little bit annoying.
But, if User Role Editor is inactive, works like a charm.

Just a quick question: I need the ability to add additional roles to my own profile as well. Is it enough to run the funktions like this? Does that affect users without admin level as well?

add_action( 'personal_options ', 'mrpu_add_multiple_roles_ui', 0 );
add_action( 'personal_options_update', 'mrpu_save_multiple_user_roles' );