A team is a group of collaborators whose contributions (when merged together) create a single piece of content that they all share ownership in. Anyone that is added to a team will be asked to sign the agreement.
There is no limit to the number of collaborators that can belong to a team. Collaborators can also be deleted from teams. However, when creating a work for hire agreement, a team should only consist of two parties - you and your contributor. Be sure to assign the person you've hired the role of "Contributor."
Creating teams makes it easier for you to complete agreements. Whenever you create content with the same group again, you can apply default ownership splits, roles, and invite the entire team to review your agreement instead of having to invite each person individually repeatedly.