Since English doesn't have a word for plural "you", I think "When did you guys meet?" Should be accepted. I'd say it's somewhat informal but it's definitely used.
English does have a word for plural you: it is, and has always been, "you."
Remember that English does have an archaic T-V system: The English word for ты is "thou" and the English word for вы is "you." We don't use "thou" anymore, and have no way of distinguishing the "respectful" second-person, but "you" still serves its former plural purpose.
has always been
If you look into the etymology, you will learn interesting things :)
I believe "you guys" is used as the default translation of the second person plural in the Chinese course. Particularly given that the Duolingo central office was comparatively involved in the creation of that course, I wouldn't be surprised to see that extending to other courses over time.
Personally, I think that you are wrong. I am not a native, but I have a lot of Russian words in my background. The sentence does not give us anything about if it is plural or not. (That's what I think)
The "вы" here certainly can be plural, which would be enough basis to allow an explicitly plural rendering in English (if the course allowed such things). The fact the verb has the reflexive particle when встретить can certainly exist without it strongly implies a reciprocal meeting. Such may be the only reasonable interpretation; I'm not sure.
I believe that SamsungApple is wrong. Вы has to be plural in this exercise because one person cannot logically "meet" without someone else being involved. You have to have at least two people for a meeting to occur.
As a result, I would really like to know how you could possibly use the singular ты with the reflexive verb.
In Katzner's Russian-English dictionary, встречаться means "to date" only in Imperfective aspect, and only with use of the preposition c + [instrumental case personal pronoun], so it might require something like когда вы с вашим встретились - which might mean "When were you dating?"
On the other hand, this verb primarily appears to mean "to meet", so "to date" is a secondary or tertiary meaning - approaching the idiomatic. That's a long-winded way of saying the c + [Instrument] might not be necessary in the right context - but absent the context, it's always a better idea to go with primary definitions - "to meet" in this instance.
[I posted this, then edited it several times over a few minutes, so if you got a prior version, sorry about that. I'm tired and made some silly mistakes.]
Editing again: After seeing some further comments in other exercises, there's an indication that the primary meaning (in the absence of any context) has shifted from "meet" to "date", at least colloquially.
I wish a native-speaker would comment on this question, because now I'm a bit confused about what it means.