Hi Jan,
Thanks for the message. As for what gets sent out: from the addon, we transmit:
Your username, password, and a list of movies. The list of movies is gathered by running a query against the XBMC database that asks for results from the title, year, and imdb id field. This is the only information that is sent, and I collect no information on IP addresses. If you want to verify, the code is on Github:
https://github.com/nmunson/script.sharethetv.
The fact remains that this site is for entertainment purposes only and in no way represents actual ownership of movies. The "collection" on your profile simply represents some lines of text sent to the site. Someone can easily "add" a movie to their collection by creating a blank movie file on their computer and renaming it to a movie name, and then scanning it into XBMC.
Lastly, if your concerned about privacy, we offer a "Delete" button on your account profile page which lets you delete your profile from the site if you ever feel uncomfortable, so that you are entirely in control of your data.
Let me know if you have any more questions.