Collective nouns are nouns that refer to a group of things, people or place. Examples include: choir, board, crew and pack. Collective nouns are singular because they refer to a single group. Example: The crew was present at the meeting. 'Crew' here represent one group of people.
Collective nouns can only be considered plural when used to refer to more than one group. When used in plural context, the noun has 's' added at the end. Example: All the boards were present at the general meeting. 'Boards' here, represents different board that were present at the meeting.
This should not be mistaken with a collective noun that has an apostrophe before the 's'. This refers to the possessive nature of the noun. Example: The choir's has a new rope.
Collective noun describes a cohesive group of many as one thing. Essentially, the 'team' is a collective noun because it is describing a team consisting of many players as one cohesive unit. A collective noun should be seen as singular unless placed in a context where it would be assumed plural. An example of this would be 'the teams at the tournament.'
Because the word team has an 's' attached to it, it can be assumed there are multiple teams of several people each that are at the tournament. Just as any common noun can become possessive, a collective noun can as well. All this would require would be the addition of an apostrophe and an 's.' The team's decision—would describe the unit as a whole and their collective decision.