First research the Colony Cluster tech on the research page, then to access it, go to the projects page. In order to use colony cluster lvl 1, you need 5 basic planets (excluding deads, U planets, assimilated or infected planets, etc..). When you cluster them they come together as 1 colony with the stats of the 5 combined, called a C1, giving more room for more colonies (as there is a limit to how much you can hold). You can then cluster 5 C1s into a C2, and so on... =]