I understand it's probably a colloquial or idiomatic expression, but is this not literally, "All together must come?" or in other perspectives "All must come together?"... literally--- I understand that in Norwegian, as written it's "Everyone" when alle sammen are used in such a manner. Just curious because there's so much spillover and false cognates between the Dutch, German, and Norwegian.
I see there's been no answer to this after 2 years, so I'll give it a stab. Yes, the literal translation is 'All together must come' or perhaps 'everyone together must come,' but one can't say that sentence as-is in English; it just sounds strange. I think there's a nuance to 'sammen' that doesn't really translate well into English, other than perhaps emphasizing the fact that alle refers to people ('everyone') and not to things ('everything').
I hope that helps a little!