Riding as part of a group is more sociable and can save you energy during a long event.
The performance advantage to riding in a group is that several people can share the responsibility of setting a manageable pace as well as sheltering their riding mates from the wind - a technique known as 'drafting.'
The most efficient formation for a large group is to ride side-by-side in pairs, with riders gradually rotating and sharing their turn on the front. How long your turn is depends a lot on weather conditions and how strong you feel, but 5 minutes is a good starting point.
Riding in close proximity to other riders does carry hazards though, so be sure to give enough space to manoeuver, don't overlap the wheel in front and be sure to keep good communication with your riding companions.