How do I occupy a rein site?

If you mean to take over a plot that is owned by another alliance and has an enemy reinforcing it then you can only take this if you can defeat the enemy that is camped there. You will need to attack with a legion that is strong enough to win the battle and knock them off.

Once you have done this you may still need to attack it again to finish taking it as you will need to do a total of 500 damage to the plot itself to occupy it.