(Note: this for the latest version as of 10-23-2012, I would assume this trick will become a moot point in the next release.)
Windows 8 changed the way the mirror virtual video drivers work. So what happens when you attempt to share your screen is the viewers get a blank screen with a mouse curser.
In order to temporarily fix the issue do the following:
- Start a join.me sharing session
- Open up task manager
- Right click join.me
- Select properties
- Select compatibility
- Place a check on ‘Run this program in compatibility mode for:’
- In the drop down select Windows 7
Research ~ A ~ holic 
This worked for me, thanks!
In Windows 8.1 on my laptop, I can share a window, but not the screen. Sharing my screen exhibits the same behavior. And the remedy described here doesn’t work apparently.
What worked for me is to disable one of the 2 graphics cards in my laptop. Then the black screen goes away.