"Don't you want to introduce us?"
Translation:Magst du uns nicht einmal vorstellen?
We use (ein)mal a lot in coversation, as a filler or because it just feels right sometimes, but this sentence is correct without the einmal, especially as a translation of "don't you want to introduce us?", e.g. if:
You and the person you’re with bump into a friend of theirs and they don't introduce you to their friend, so you say “magst du uns nicht vorstellen?” It's perfectly fine without the einmal in this case.
I thought of a context in which the einmal would make sense, but in my opinion the English sentence doesn't indicate that this is what's meant:
You talk to someone about a friend of theirs who sounds really awesome and you’d like to get to know that friend. - “Magst du uns nicht (ein)mal vorstellen?” = “Don’t you want to introduce us sometime?"
(Note that even in this context the sentence is still correct without the einmal, it just sounds more complete with it.)
It comes after the direct or indirect object.
See here, search down on "placement of nicht": http://www.dartmouth.edu/~german/Grammatik/WordOrder/WordOrder1.html