"Teams earn control of a capture point by having ships within the designated control area;
additional ships within the boundary will increase the speed of capture: one ship within a boundary will capture an area in 60 seconds;
two or more will capture an area in 40 seconds. (For Clan Battles, these values are 38 and 25 seconds, respectively.)"

http://wiki.wargaming.net/en/Ship:Game_Modes#Domination