My guess would be that both patterns are possible but it's hard to make a definitive statement because the situation doesn't occur often in the literature. I only vaguely remember seeing it with a plural verb once. I think it was in the Vulgate (so not classical Latin).
In general (that is, not in the imperative specifically, but in other moods) singular collective nouns take singular verbs. But they occasionally take plural ones. An imperative is a little different, though, because the vocative that accompanies it isn't technically its subject.