Have one person do all the fastpasses, especially if anyone in the group has never made them before. I learned the hard way when a newbie in the group cancelled all my hard-to-get fastpasses (which I couldn't get back).
Also allow times for some group separation. It can be hard to move through the parks in a large group, especially day after day. Doesn't have to be for long, but if some want to shop while some go on rides then there should be flexibility.