You should always have 1 heal 1 tank 2 dps in both groups. You also want to place the tanks to the middle, casters to the edges and non-casters wherever in-between them. Why do it like this? With the tanks in the middle, it's easier to adjust for Twintania, since you don't have to worry about overlapping missiles since tanks don't get hit by them. The casters can sprint so they can take the furthest distance from stack point. Some groups place one guy to the same spot as where the tank is to allow for a bit more of moving space, although it causes some dmg to the tanks.

Tbh it's mostly what everyone else has been saying; know what is happening and when it's happening. Red? Scout. Green? Go. Twin in front/behind? Adjust. No non-tanks hugging you? Good.