to share:
click on the "share" button on the toolbar. drag the FILES ([strike]NOT FOLDERS[/strike] if you share a folder, only the files directly inside of it will be shared. any folders inside of the first will not be shared.) into the box marked "drag selected files and folders here" or whatever it says.
highlight the names of everyone on the list below it that you want to share with. you can select which people can see which files. this allows you to share porn with your buddy rick, and only show your grandma the beatles mp3s you have.
after selecting all the names, hit "share"
if you are trying to share too many files, you'll be waiting for a few minutes. otherwise the sidebar will just disappear. click on your name on the left sidebar at the top of the list of "groups." if you're succesfully sharing, it should list all your files there.