A work team consists of the client owner for the contact associated with the work item, anyone assigned tasks, emails or notes relating to the work item, and anyone who is manually added to the work team.
You can view the work team on the work item's Details tab, along with a column detailing each member's involvement with the work item. If this column is blank, the team member has been manually added.
If you have Time & Budgets turned on in your account, additional details on team member involvement can be found on the Time & Budgets tab. These details include the amount of time each member has been budgeted to spend on each task or role, as well as their actual time spent, etc.