We had a Yabla lesson some time back about false friends in large numbers, where we discussed how a million is eine Million in German, but that bigger numbers get confusing, since a billion is eine Milliarde but a trillion is eine Billion.
Smaller numbers like a hundred or a thousand are treated like standard adjectives in German: they are written in the lower case, and the plural nouns they modify remain the subjects of the sentence:
Über hundert Leute der Bundesregierung dürfen sie einsehen.
Over one hundred people in the Federal Government are allowed to see them.
Caption 15, Deutsche-Welle-Nachrichten: Massenprotest gegen TTIP
Also, beim Finale waren halt auch tausend Leute.
So, a thousand people were at the finale.
Caption 29, Eva Croissant: Interview
As you see in the two examples above, hundert Leute dürfen and tausend Leute waren, the adjectives hundert and tausend modify the subjects of the sentence, which remain plural and require the plural conjugation of the verbs.
However, larger numbers are treated differently in standard German. If you have a million of something, the core of the subject remains the term "a million" and the verb is conjugated for a singular subject, even if the subject appears plural.
Rund eine Million Menschen wird in der Stadt erwartet.
Around one million people are expected in the city.
Captions 23-24, Rhein-Main-TV: Feier zur deutschen Einheit in Frankfurt wird gigantisch
Note that in the above, the verb werden is conjugated for a singular subject as wird, rather than the plural werden that you might have expected. That's because the core subject of the sentence is the singular eine Million, not the plural die Menschen.
As a rule, it's just good German to use eine Million and larger numbers in the singular as that is standard usage, but an article on the topic of the German Duden dictionary states that der Plural wird aber auch akzeptiert. So at least if you say eine Million Menschen sind instead of the standard eine Million Menschen ist, then it's not really a mistake, it's just an accepted variation of the standard German.