I'm speaking from a view point of never having DM'd before (sorry), but is it possible to create a situation where the newer player's PC's can branch off in a one shot type setting? It could be within or even before the current timeline of your main campaign if that works.
Maybe let the more experienced players who will be attending jump in as new/rerolled characters or player controlled npc's to help things along? Or even as their own characters guiding the new group if that fits the setting better. It might be better than facing your new players with a brand new stat sheet / class / character rp / play style?
If you need to keep your timeline straight, could there be a setting based reason for the present/inexperienced pc's to be temporarily seperated from the group, sending them out on their own with the aim of reuniting their party?
A forced situation of hidden doors/ traps or a plot device of 'we need this item/enviroment trigger/ knowledge/ person to continue, so can you all backtrack and find it?' might work.