When it comes time for your session, I am pretty flexible on what happens when.  For example, We can start with the whole group, or if some family members are running late we can start with the smaller family groupings.  I usually start with the family that has the youngest children, because they usually have the shortest attention span.  I make sure to capture all the different family groupings, so each family gets a little family session to themselves, singles of each kid, sibling groupings together, cousins together, big siblings together, Grandma & Grandpa together, the original family together (what I mean by this is Grandma, Grandpa, and their immediate children), and what ever other groupings you would like.