Project managers can specify the team for their projects. The objective of this is two-fold:

When assigning tasks to resources, you can either choose to display all resources within the company or only those that are in the project team. This is especially convenient when you have many resources which make it difficult to promptly locate a resource from a very long list.
You can also control permissions to the project team members, which could be revised / modified / changed during the course of the project.

Project Team dropdown is available when the option When assigning tasks to user show all users, not just users in project team at company level (Settings > Company, under 'Projects & Tasks' section)
is unchecked.

To assign team members in a project:

Edit the project by clicking on the sign that you see besides the project name.
Under Advanced tab, click Project Team drop-down list.
Select the users that you want to add.
Click OK.
Once you define the other mandatory project attributes on the New Project window, click Submit. The project team members are assigned to the project.

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