I want to be able to say that I was co-responsible for a thing, e.g. "I was co-responsible for the company's internal annual conference," but that's not an accepted word in standard English, right? So how should I phrase it instead?
It is fashionable at the moment to use 'co-' as a suffix. I recently saw a new healthcare complex advertised as having 'minor centres co-located with the main hospital'. It is quite common in corporate contexts to invent new words by adding prefixes or by turning nouns into verbs, or both. So 'co-responsible' is definitely something a native English speaker might say in this kind of context. But it's always dangerous to play around with things like this if English isn't your native language so I think the above suggestions are better. Tbh corporate jargon can sound quite silly at times even when it's entirely idiomatic within a given context. For example I might say I can 'uniquely mesh resource-leveling infomediaries' and be praised by my corporate colleagues, but it looks like gobledegook to most people.