The concept behind opportunity teams is the same as account teams: a group of users working together for an opportunity.

Click on Setup | Feature Settings | Sales | Opportunities | Opportunity Team Settings and flag Enable Team Selling.

Member roles have the same values of account teams (Setup | Feature Settings | Sales | Team Roles), so consider this when changing the picklist values.

You can add a default team, an account team, or add new users:

The only access level you can open to the team's members is opportunity access:

The concepts of member creation and editing are the same as for account teams.