The rule is as follows:
If the group acts in unison, use a singular verb:
EX: The team [it] is late.
EX: The family [it] is coming over tonight.
If the group acts individually, use a plural verb:
EX: The team [members] are late.
EX: The family [members] are coming over tonight.
It's common for British English speakers to use "are" and for American speakers to use "is". The reason being, British English speakers tend to view the group according to the individual contributions of its members; i.e., the team [members] are, whereas American English speakers tend to view groups as a collective whole; i.e., the team [it] is. In short, speakers who share the same language will differ in how they see the world around them; e.g.,
USA: The team is a unit.
UK: The team are individuals within a unit.