Riding with the slow group initially and learning how to ride with a group is great advice because the group dynamic alters your riding experience both in efficiency and psychologically. The average speeds are usually there to give riders a sense of how fast they will generally be going and remember that if you ride in a good paceline maintaining higher speeds is easier because of the drafting factor. Psychologically group rides motivate you to push yourself harder when the pace picks up, and having company when you're suffering up a climb is pretty nice too.