rwmb_before
This action runs before all fields in a field group is displayed. It's usually used for outputting custom HTML.
do_action( 'rwmb_before', $meta_box );
It takes 1 parameter: the instance of RW_Meta_Box
class for the current field group.
To run this action for a specific field group, use a variation of this action:
do_action( "rwmb_before_{$field_group_id}", $meta_box );
Examples
This code wraps a field group with ID event-details
into a custom div:
add_action( 'rwmb_before_event-details', function( $meta_box ) {
echo '<div class="my-div">';
} );
add_action( 'rwmb_after_event-details', function( $meta_box ) {
echo '</div>';
} );