Oh easy way is if you have a friend from this group just go to there page and under there groups/communities they joined will have the septemember group and will have a join button next to it.
On the top right under Prengnancy Sept 2012 babies community it should say about this forum then next to it should say join this forum in blue lettering... if that's not there then idk how else to join and hopefully someone else will know how. But I hope that works for you.