"Die Menschen reden und gestalten mit."
I find this sentence weird. I thought "mit" was a preposition. So can it be used to mean "together," as in a translation suggested by Duolingo, or to suggest the same meaning as the prefix "co-" in "co-designing?"
Both are compound verbs actually, 'mitreden' and 'mitgestalten', the prefix mit- works the same as co-, as far as I know. Although I'm not sure if duolingo really meant 'mitreden'. It's possible they mean the talk and therefore co-create, co-design whatever they're talking about. If that's the case, 'mitreden' might be better.
So "mit-" can also be the prefix of a separable verb? I had already encountered a bunch of separable verbs in Duolingo: aufhören, abfahren, ausgehen, just to name a few. But this is the first time I met something starting with "mit-."
What you said just helped clear things by a lot. Thank you! :D
Yes, it is one of those. Usually it's something like co-, but sometimes closer to 'join in', like a conversation for 'mitreden' or a game (mitspielen, join the game, join the playing). Mitspielen can also be 'to play along' for example.