A collective noun, also called a group noun, refers to a group of persons or things. Examples include group, number, majority, team, and many others.
In American English, when individual members of the group noun act individually, you should use a plural verb. Study this example:
A majority of courts have followed Roe v. Wade. (Here, the courts have acted individually, in separate cases, not as a single group or mass. Thus, the verb have followed is plural.)
A group of Senators has decided to introduce the bill. (Here, the group acted as a unit. Thus, the verb has decided is singular.)
The jury have reached their verdict.