Whether a collective noun, which is singular in form, is used with a singular or plural verb depends on whether the word is referring to the group as a unit or to its members as individuals. In American English, a collective noun naming an organization regarded as a unit is usually treated as singular: The corporation is holding its annual meeting. The team is having a winning season. The government has taken action. In British English, such nouns are commonly treated as plurals: The corporation are holding their annual meeting. The team are playing well. The government are in agreement. When a collective noun naming a group of persons is treated as singular, it is referred to by the relative pronoun that or which: His crew is one that (or which) works hard. When such a noun is treated as plural, the pronoun is who: His crew are specialists who volunteered for the project. In formal speech and writing, collective nouns are usually not treated as both singular and plural in the same sentence: The enemy is fortifying its (not their) position. The enemy are bringing up their heavy artillery.
When the collective nouns couple and pair refer to people, they are usually treated as plurals: The newly married couple have found a house near good transportation. The pair are busy furnishing their new home. The collective noun number, when preceded by a, is treated as a plural: A number of solutions were suggested. When preceded by the, it is treated as a singular: The number of solutions offered was astounding.
Other common collective nouns are class, crowd, flock, panel, committee, group, audience, staff, and family.