To add to the above, the reason (as pointed out) is because "it" is singular, and therefore the verb and subject need to agree. That means "it was" has to be used.
Alternatively, you could say "they were just a couple of friends", in which case the subject is "they" (plural), so the verb needs to agree and take the plural too.
So here are a couple of other examples:
A sudden noise startled him. But it was just the same old couple arguing again.
Coming towards him, he spotted two dark figures, and tensed. But they were just a couple out for a midnight walk.
What is important to notice here? The "it" or "they" references something in the previous sentence, either the noise (it, singular) or the two figures (they, plural). This means that context and what went before is often important.
Things can be complicated slightly too by the use of collective nouns in BE, where a team (singular) can be treated as plural because it is viewed as being made up of a collection of individuals.
Anyway, you can often rephrase while retaining meaning, so in this case you could also say:
Calling them a camera crew would be a stretch too far; they were really just a bunch of friends.
HTH