Android:

1. When you access the feed (last tab on the bottom) Click on the "+" in order to open the option to add a post in the community


iOS:

1.  When you have tapped on the "Communities" tab and you see a list of communities, in the top, right hand corner there will be a pencil and paper icon.  Tap on that icon and you can create a post, and assign it to a community without even having to enter.


2.  Inside the community, at the top, right hand corner the pen and paper will still be there for you to create a post.  There is also a space at the top of the community that looks like a status update, you can also create a post here.