Typically guest accounts have fairly restrictive access to the system, especially on publicly accessible systems. Groups are simply groupings of users. They are primarily used to ease system administration. For example, instead of having to assign access to a new hard drive to the forty accounting users, an admin just has to assign the accounting group the access. Even special privileges can often be assigned by group, such as the ability to manage a set of programs or system functions like printing.
Most modern systems allow accounts to belong to more than one group.