its one sutta a week, so if you didn't get in it that week it's closed, at least that how it is for now. unless there is a better system implemented at some future point.
we're in the sutta nipata now, i have been doing them in chronological order. the sutta numbers are in the beginning of each thread. i would assume if you wanted to join in for the sutta that week , you would just open the thread and read the sutta, i don't know. i get them all from access to insight.