I am an active OSM contributor and I've always had some geofictitious worlds in my head that meet the requirements here (modern world fiction) and I would be glad to join the project for mapping my ideas in your world.
I tried to join like several years ago but my account was terminated after hours, for some unknown reasons, without me being able to do a single commit. So I hope the project is still active and that I will be able to participate in it.
If I understand, I must prove my skills by populating an area of a common zone with JOSM, create some test cities etc., and then if I am active enough I will be able to take possession of an available country.
I do not know how to get in touch with other members in case there are some mandatory ways of mapping, like some unwritten customs I should know, in addition to those written on the wiki home page etc. but anyway if I do any mistakes please let me know by PM before banning me !
I hope you understand my motivation in the project. See you at Commonia's best beach resorts !