Team Foundation Server Workgroup Edition that you receive when you purchase a Team Edition with an MSDN Premium Subscription provides for FIVE named users and does not use the CAL model.
To support larger teams, you need to purchase a license for Standard Edition (full retail price: US$2799) and CAL licenses for each user (~US$500).
You would find it to be financially beneficial to purchase that software through Volume Licensing, which provides discounts when purchasing five or more licenses.
Other related prices:
MSDN Subscription includes versions of all servers, everything for 1 person. Includes 1 Team Foundation Server Workgroup Edition of those includes 5 licences. Price around US$6,500