Question about group cruises?

So how do they work? I will like to set one up with some coworkers, I know you need 8 rooms to qualify for group travel.

Do you get a good discount for doing group travel? How does payment work? Is the leader of the group responsible for payment or does each individual person submit their own payment to RC? Do you have to do all the excursions with the group, or can we all branch off into our own separate thing?

Thank you.