You may apply for an invitation on their home page.
The better way though is to look for friendly people, look up their CC, check if they are active, check if the owner is active (date last played...) and ask him for directions and to invite you.
This is because an active owner is most important for any CC, and there are too many out there which are dead CCs with inactive owners.
Good thing is, you may join for free and leave anytime, for free.
Have fun!