For these early stages, I think it's wise to keep everyone leveled equally until they reach level 4 or 5.  After that, I believe the minimum level difference between the highest leveled character and the lowest leveled should be 2 or 3.  So if someone is level 8, anyone level 4 or 5 will be leveled up between chapters to close the gap until they're 2-3 levels within the level 8 character, level 5 or 6.<br/><br/>I'm open to suggestions however.