For a detailed description of the product go to here.

How many licenses do I need?
You must have at least as many GroupCaelndar licenses as you have active Microsoft CRM licenses. If you do not, GC will not function for any user. For example, if your organization has purchased 100 Microsoft CRM licenses and has only 25 in use, but plans to put another 5 in use over the next few months, it is advisable to purchase at least 30 licenses so that your users will not see an interruption in GC functionality. In this example, the minimum number of licenses you need is 25.