HUMANKIND

To get this you need to have a friendly status with the Independent People you come across in the Classical and Ancient Era. They are AI player that automatically spawn on the map, as long as there is space for them to spawn. If you claimed all territories on your Island, they won't spawn there any more.

There are two types (of biases) of independent people, friendlies (peace symbol) and hostiles (thunder symbol). The first one are just roaming around, while the latter will actively attack you or even try to attack your city, especially on higher difficulties.
(Their biases are personally to your empire. A friendly IP might be hostile to another empire.)

If an IP is friendly to you and close to your borders they will like you more over time, but we can speed up the whole process with any IP by spending influence or money on them. You can see this when you select their city in the lower right corner. There is a bar which you will need to fill up. Once it's far enough you can trade with them or hire their armies (for the other three achievements).

To assimilate the IP you need to wait until their bar is completely full. But be aware: other empires might also have them in their Sphere of influence or they are spending resources to gain their trust. Therefore you need to keep on eye on your status with them all the time and keep on spending resources to get the assimilate option as quick as possible.

Once you have it, assimilate them, repeat it with five other IP and the cheevo is yours.

If you run out of space: you can liberate your own city, making it Independent People again, which will let you repeat this over again.