Hi Brunschot Reklame,
In case you want to add a seat to the existing CC Team account, You can do that as you will be charged only after 30 days or in the next billing cycle.
You can get detailed information in :http://www.adobe.com/products/creativecloud/teams.html
http://www.adobe.com/products/creativecloud/faq.html
Regards,
Rajshree