When you reach the 16 level, you can join to the guild and participate in the guild challanges. 

Participating in the challanges, you can get a lot of rewards! Thir amount depend on the number of points that you got. The rewards will open gradually - the more points you get, the more the rewards you receive!

   
To start, you only need to build the Exploration center and select group that you want to join. Also, you can create your own group and invite your friends to join you! 
   

You can also check this little tutorial here:

https://www.facebook.com/ci2tropicalstories/videos/295440588633111