Groups in Plone
Users can be in groups - managed via users and groups administration in plone setup. Groups can be given particular roles if need be - all users in that group will gain the given role.
It is very rare that only one user will ever have a given role. More likely, there will be a group of users with the same role in the portal - and even where there is currently only one user (say, only one site manager), it often makes sense to cater for the possibility that there will be more users later on. Since users can be in several groups at the same time, you can get fine grained control over permissions and classifications of users using this mechanism.
The previous page showed how to make new roles in the portal. Using the users and groups administration tool in Plone setup, you can easily assign roles to groups - just tick the relevant boxes and save. Then, click on a user to assign him or her to the relevant groups. Once in a group, the user will gain all roles assigned to that group in addition to any roles assigned only to that user.
It is also legitimate to use groups to logically group users without assigning any special roles. This has the added advantage that members will gain a shared group folder (if turned on in the 'portal_groups' tool). It can also be used to assign local roles to groups - the topic of the next page.