Bamboo groups are used to specify which users will have global permissions and plan permissions. They can also be used to specify which users will receive notifications about a plan's build results. You can create and delete as many groups as you need. You will typically create at least one group per project.
A special group called bamboo-admin is automatically created when you install Bamboo. Members of this group have Bamboo administration rights.
Read more about managing groups for your users: