it's pretty simple. the first person to join a rookies can claim it as normal, 13 post, 58 post, charity, first hoh, etc. and then people join to participate in that afterwards.
13 post rookies = your initial post is your only post
58 post rookies = 1 post per player per day
20 post castings = your initial post is your only post (you can PM to keep your health up but your checks won't go up)