User blog comment:Rainsplash987/Not sure whether to advertise this or beg you guys to join.../@comment-16166566-20130718233655/@comment-5342796-20130718233814

It means that if someone has to leave or something happens, you'll get to join. You can also sub, meaning if someone is going on vacay or something long enough to disrupt the schedule of the collab, you get to write it. :) You don't have to join, just an option.