You just have to use ki-list command to add player one by one or all with @a parameter.
About JSON instead of YML, I prefer JSON because of the file generation way and because it's more readable.
I hope it answer your question and do not hesitate to ask me again if needed.
Version: 1.2.1
Very simple and straightforward to set up and run. I was looking for something for my SMP community to do for an event and I found it in this plugin. Great job!
My suggestion would be to add a way to customize the rewards given to the player(s) after they complete each age. Other than that, a good mini-game for an SMP community!