How do you say that John and Marie are "one of the representatives" of this company? Is it correct since you have John and Marie as both representatives?
There isn't a good way to combine multiple people with the phrase "one of the representatives." If you think about it, by describing the pair as "one of the representatives," you are suggesting that the two people are acting as a single representative. It doesn't make physical or logical sense. Here are the ways you can express the idea:
John and Marie are representatives of the company.
John and Marie are two representatives of the company.
John and Marie are two of the representatives of the company.
The three examples, above, are equivalent in meaning. You could theoretically say the following:
The Hanson family is one of the representatives of the company.
It's grammatically correct, but I wouldn't expect to hear a native speaker say it. The word "family" is singular in this context, but it implies multiple people and "representatives" usually implies individual people. On the other hand...
The Hanson family is representative of the Human Race.
works well... but it's also not what you're asking about.
If there are multiple representatives, they can't be "one of the representatives" unless they are sharing the role and only count as one 'person'.
If you know the number, as in this case, you can say "two of the representatives". Or "three of the representatives", or whatever, as appropriate.
If you don't know the number, but it's more than one, you can say "some of the representatives." You can also use that if you know the number but are not stating it at that point. If the number is obvious from immediate content, it's not wrong to say 'some', but it may come across as odd.