A user can be directly assigned permissions, through a team, for a circle, or a user rights groups, which determines their level of access within that particular site or circle.
For circles, it is however highly recommended to first of all create circle user rights groups and then create suitable teams that can be assigned the user right groups. Users can then be added to the respective teams as it suits the situation. In this way it is also easier to assign work and mention users in comments by simply using the team name. See more on teams here.
The permissions or rights a user can get depends on the user license that they have. A user will only take up one user license for the entire Site.