Give Users More Than One Role

add_action( 'edit_user_profile', function ( $user ) {
    if ( ! current_user_can( 'promote_user', $user->ID ) ) {
        return;
    }

    $roles   = get_editable_roles();
    $primary = reset( $user->roles );
    unset( $roles['administrator'], $roles[ $primary ] );

    if ( ! $roles ) {
        return;
    }

    wp_nonce_field( 'wpide_extra_roles_' . $user->ID, 'wpide_extra_roles_nonce' );
    echo '<h2>Additional roles</h2><table class="form-table" role="presentation"><tr><th scope="row">Also give this user</th><td><fieldset><legend class="screen-reader-text">Additional roles</legend>';

    foreach ( $roles as $role => $details ) {
        printf(
            '<label><input type="checkbox" name="wpide_extra_roles[]" value="%1$s" %2$s> %3$s</label><br>',
            esc_attr( $role ),
            checked( in_array( $role, (array) $user->roles, true ), true, false ),
            esc_html( translate_user_role( $details['name'] ) )
        );
    }

    echo '<p class="description">The main role is chosen in the Role menu above.</p></fieldset></td></tr></table>';
} );

// profile_update runs after WordPress has saved the main role.
add_action( 'profile_update', function ( $user_id ) {
    if ( ! isset( $_POST['wpide_extra_roles_nonce'] ) || ! wp_verify_nonce( sanitize_key( $_POST['wpide_extra_roles_nonce'] ), 'wpide_extra_roles_' . $user_id ) ) {
        return;
    }

    if ( ! current_user_can( 'promote_user', $user_id ) ) {
        return;
    }

    $user     = get_userdata( $user_id );
    $roles    = get_editable_roles();
    $primary  = reset( $user->roles );
    $selected = isset( $_POST['wpide_extra_roles'] ) ? array_map( 'sanitize_key', (array) wp_unslash( $_POST['wpide_extra_roles'] ) ) : array();
    unset( $roles['administrator'], $roles[ $primary ] );

    foreach ( array_keys( $roles ) as $role ) {
        $has = in_array( $role, (array) $user->roles, true );

        if ( in_array( $role, $selected, true ) && ! $has ) {
            $user->add_role( $role );
        } elseif ( ! in_array( $role, $selected, true ) && $has ) {
            $user->remove_role( $role );
        }
    }
} );