Learn Russian: Среди гостей находился сообщник.

In Russian, the end of the sentence is typically reserved for the "new" or most important information. Because Russian doesn't have articles like "a" or "the", putting сообщник at the end implies indefiniteness—it translates to "an accomplice" rather than "the accomplice". The sentence sets the scene first ("Among the guests...") and then reveals the new piece of information.

How would the meaning change if we said Сообщник находился среди гостей?

If you put Сообщник at the beginning, it becomes the "known" information (the Theme). It would translate as "The accomplice was among the guests." The focus of the sentence would shift to answering where the already-known accomplice was located, rather than revealing his existence in the crowd.

Why use the verb находился instead of simply был?

While you could absolutely use был (Среди гостей был сообщник), находился sounds slightly more formal and descriptive. It comes from находиться (to be located / to find oneself) and emphasizes physical presence in a specific space, which sets a perfect narrative tone, like in a detective story.

A quick refresher: why is гостей in the genitive case?

The preposition среди (among / in the middle of) always requires the genitive case. Here, the noun гость (guest) becomes the plural genitive гостей.