I have done a great deal of group presentations in my life. Most of them involve use of Powerpoint slides in my professional career, which allow you to easily reference information so you do not need to solely rely on your memory. However, sometimes you need to make the off the cuff remarks when responding to questions and other times you need to do a more spontaneous presentation of information in life (e.g., best man speech, accepting an award, presenting to a group over a teleconference).
As alluded to earlier, retrieval of information is enhanced when the person is very familiar with the topic area. This is because brain cells form stronger and stronger connections with each other the more well-known the information is. The technical name for this is having strong synaptic connections. Synapses are the connections between nerve cells and those bridges become strengthened when learning is enhanced, improving the memory trace.