I've got a question.

[Derfasciti]

Ah, so at least for some of them, you can just put in your two cents on the specific question and the votes are tallied and majority rules?

How/who/what determines the end of the scenario?

[Franconicus]

The creator of the stoty makes the rules. We had stories where the first or the third vote counted. Today, the mojority vote is most popular.

[Kraxis]

Everyone can leave a post in the open stories with choices.

However, it is generally a good idea to make an argument for the choice (if for nothing else than to prove you are serious and not just nuking the story).

First post decisions (where the first poster gets the choice) is good for faster paced stories with few choices for each chapter. It gives the readers a sense of urgency, which might cause them to slip a bit and make bad choices as they scramble to be the first (their choice is the right one after all, and it is important that it is chosen).
However, it also leads to a relatively stagnant thread in between the author posting and the choice is made. There is little need to post as it can't really affect the path taken. Hence the speed. Also the urgency can lead to contradictory choices picked in successive chapters.
Take a look at Interactive History III: Antiochus' Dilemma for a good insight to this.

The majority decision stories can on the other hand grow immensively, and can drain the author, as he needs to take in all the choices, opinions and ideas. Also at times there are ties. Further, with multiple choices you can end up with odd combinations as the most posters haven't got the same idea of what is right (if people are too much in agreement I consider the choices to have been bad on my part).