Newbies join mid campaign

So here's the deal. I am hosting a D&D campaign for my group of 3-4 friends. Lets call it group 1. Now while making the campaign I talked to some other people I know and turns out they're into the whole D&D thing. So I offered for me and the two of these new folks along with one of the members of group 1 to meet up and play the same campaign but from the beginning. Problem is, the rest of the Group 1 found out and want to join. Now what do I do? Do I start the campaign over? Then the members of group 1 will be bored. But if I allow the newcomers to join in the game half way through they will either be under-classed or miss crucial plot details. I am lost. Help me please.