Collective nouns refer to multiple people or things as a unit. Collective nouns should usually be treated as singular, so pronouns that refer to them should be singular as well.

The band has been together for four years, and they have played at Hooper’s six times.

The has played at Hooper’s six times.

When members of a group are acting individually, however, a collective noun can be treated as plural:

The band wrote their autographs on a poster for me.

The audience set their chairs in a circle around the band.

If you want to avoid having to treat the collective noun as a plural, you can reword the sentence:

All the band members autographed a poster for me.

The fans who brought chairs set them in a circle around the band.

When a collective noun is treated as singular in one part of the sentence, don’t treat it as plural elsewhere in the same sentence:

You can usually reword the sentence to avoid this problem:

The band, which has been together for four years, autographed a poster for me.

Remember that writing, unlike casual speech, gives you a chance to correct yourself before anyone else reads your work. Your readers will expect you to get it right. Take the time to revise your writing so that the pronouns agree with their antecedents.