Some of the advantages of group decision making are sharing of information and creation of synergy, while the disadvantages include taking a longer period of time and diffusion of responsibility. Whether the advantages outshine the disadvantages or not purely depends on the particular group in question.
People in a group are likely to give different perspectives concerning an issue, and in so doing discuss and come up with the most relevant solutions. As the group coordinates by sharing information, it is likely to create better awareness on an issue and capitalize on each member's expertise in dealing with it.
However, a group may also become less effective as some members may shift blame when a decision does not result in a good outcome. Having all members participate and contribute regarding certain issues may equally take a longer period of time and create misunderstandings.