"Re-building" an established campaign

Okay, so here is a problem I have on my DM plate: I have a long-running, multi-year, deeply roleplay immersive campaign that was begun in 3.5 DnD.

Yeah, duh, it's "Elven Quest"!

So, the wonderful engine of the story and awesome, broad roleplay and character development has ground to a halt, because the group changed a little, we are down to at best 4 players, on a very good day, and we are exploring low-level 4e with an overlapping gaming group. (By overlap, I mean some players are in both groups).

I want the campaign to continue, but to do that, one of things I must consider is adding new players to a story which has a LOT of background, and possibly having them feel like someone is telling inside jokes all the time, and they are left out.

So, I'd love to hear any ideas from people as to how they might handle it... and "end the campaign" is not an option!