
A group of riders in a well organised bunch (also known as a peleton) uses much less energy than the individuals riding alone. Riding in a group is also much more social than riding alone.
There are some potential hazards though: sharp braking or veering can bring down other riders. Maintaining a consistent speed and line are a must.
You should practise riding in small groups before riding in a larger group where speeds tend to be higher and the margin for error is smaller.