OFFLINE

If you have an empty city slot, you can actually take someone else's sub city.  You can tell if it's their sub because it has an orange flag in the bottom right corner on its icon on your map.  You can't take someone else's main city.
To take a city, you have to reduce its loyalty to 0. Ten attacks in 1 hour. You can only attack 5 times in a day. So you will need someone else to attack it 5 times plus your five.