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I understand 一緒 cannot be substituted with 共に given how flexible 共に is and how people-specific 一緒 is.

But can the reverse be true since 共に can also be used with people?

例:友と共にパーティに行った。

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「共{とも}に」 and 「一緒{いっしょ}に」 certainly share the meaning of "with ~~".

The important difference, however, is in the formality levels of the two phrases. While 「共に」 sounds formal and slightly stiff, 「一緒に」 sounds informal and conversational.

In our informal daily conversations with family members, coworkers, friends, etc. the vast majority of us would almost exclusively use 「一緒に」. 「共に」 simply sounds too serious for daily use.

What this means is that if you used the two interchangeably, your "new" sentence is bound to sound awkward because that is what inevitably happens if you change just one word out of a whole sentence to another with a different level of formality. Your new sentence will just lack the good balance with one of the words sticking out as a poor word choice.

Your sentence:

「友{とも}と共にパーティに行{い}った。」

sounds borderline okay because 「友」is already stiff compared with 「友だち」. Stiff-on-stiff is alright.

If you asked me, however, if any native speaker would utter that sentence verbatim in real life, I would honestly reply that the chances would be very close zilch. It is far more natural to say:

「友だちと一緒にパーティーに行った。」

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