If you try and assign a document to a group (or selection of groups), that have more members than you have licenses then, you will see an error in your Event Logs (accessible from Central Admin). As below.

This error will be raised when any of the timer jobs run (configurable from Central Admin) and also when you attempt to 'Process Site Reading Tasks' from the site in question.

To learn how to get more licenses for either (users or sites), please see this post : http://support.collaboris.com/entries/20638517-more-licenses

Please note, DocRead will always calculate the licenses needed before assigning tasks. This means that you wont get some users in a  group receiving tasks, when others can't because the license has exceeded.