Warum seid ihr denn nicht bei den anderen?

English Translation

Why aren’t you with the others?

Would be “So, why aren’t you with the others?” correct?

It is difficult to translate the “denn” with a single English word.
I would approximate the meaning with

"Why arn’t you with the others, (it would be more logical/I would expect it).

I doubt that “so” has the same underlying meaning, but maybe my English is not good enough.

For “denn” is this context, see also:

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As a native German speaker, the word “denn” in this context is not strictly necessarly and only adds some subtlety but is often omitted when being translated to English.

I think I would translate the German sentence as “But why aren’t you with the others?” if you want to keep that subtle detail.

If someone uses the word “denn” in this context, it implies that this someone assumed that of course you would be with the others (after all, why wouldn’t you be?), but seeing that you’re obviously not with the others, surprises this someone, which prompts him/her to ask you the question.

Your translation “So, why aren’t you with the others?” essentially means the same thing, but in my opinion lacks this subtle conveying of surprise and that an assumption existed to begin with, which then turned out to be false.