tell the cast member loading the attraction the total number in your party, they will split you up. They will know the best configuration, and they will divide you appropriately.
(a lot of times, larger parties will try to split themselves, thinking they're making the CM's life easier. The reality is the opposite. Consider the following scenario:
CM: How many?
A: 4
CM: Ok, you can take that last empty row in that vehicle.
CM (to next person after 4): How many?
B: 3, But we're with them *points*
CM: They told me 4, they must have wanted to get rid of you. You'll have to wait for the next vehicle )