You choose your own seat.
The way the seating groups work is, the higher your seating group is, the earlier you get to to board. Usually they board like A1-A30, then A31-A60, then B1-B30, B31-B60, etc. I think the people with A1-A30 are usually either business class or people who reserved their tickets WAAAY in advance.
Regardless of seating group you get to sit where you want (assuming nobody else is there) but the later you board, the more seats will already be taken. If you're in Group C you're pretty much taking leftovers.