Sentence context identifies whether you use plural or singular verbs with collective nouns. A flock of birds is on the ground feeding when a cat jumps out; the birds fly away in unison, using a singular verb: "The flock flies as one." Any action that a collective noun does in unison demands a singular verb. "After practice, the team goes inside." Their unified action uses the singular verb "goes." "Ms. Smith's class takes its test today" indicates the class, in unison as a group, tests.
When the members of a group are performing individual actions, the verb is plural. If those same birds chased by the cat decide to act against nature and fly individually, the verb is plural: "The flock are flying in all directions." If committee members are expressing differing opinions, you would say "The committee are disagreeing." If cast members practice, but each is learning an individual part, you say "The cast have been practicing their lines." For better clarity, you can add a word such as "members" -- "committee members" or "cast members" -- to the sentences.
Perhaps the best way to illustrate plural vs. singular usage is for you to imagine a group that acts individually, then in unison. An orchestra tunes individual instruments; we therefore say "The orchestra are tuning their instruments." Anyone who hears an orchestra tuning up can say they are making sounds, but not unified ones. However, once tuned, the orchestra plays a single, unified piece: "The orchestra is performing Beethoven's Symphony No. 5." The same collective noun can have both singular and plural verb agreement, depending on the group's actions.
Some collective nouns are naturally plural, such as "families"; some collectives, such as "police" and "people," are usually used in plural verb agreement, as in "Most people are forgiving" and "The police are very efficient." If you are at all in doubt as to the plural or singular nature of your collective noun, a dictionary will tell you which usage is correct.