The Team is all of your Contacts that are associated with a particular Project. They can be added either manually in the Team screen or automatically simply by adding a Contact to a Task in the Timeline.
A Group is a subset of your Team. For example: Groups might be “Designers”, "Cast" or “Full-time staff”.
Groups are used to add a bunch of people to a Task at the same time or they can act like a distribution list to send Filters and Paperwork to a whole bunch of people at once. Each Team member can be part of multiple Groups in a Project.