We have a family blog using WordPress for Multiple Authors. Some family members don't contribute very much. It was suggested to create a game of "Tag" where after one person writes an article they get to choose/Tag the next person to write a new article. I think it would be best to keep track of who has been tagged each round and only allow people to be tagged once per round until all people have been tagged (then start a new round). The script would need to display who can be tagged, and then send an email to the person being tagged.
People would need to be able to opt-in before the round starts. If they aren't opted-in then they can't actually read the articles for those playing in that round.