Sharing/working on same files

[GitsLM]

Hi guys, I thought that maybe one of you have heard about an app with whom I can share files with one or up to three users. It should have encrypted transfer and it should not make any files publicly available. And it should be able to bypass firewalls like e.g. Zonealarm since both parties are behind firewalls. Does anyone know of something ? The best would be a open source one but I am interested in all possible solutions.

I now that I could use a ftp server and set up those users but I am still searching for an easier solution.

Thanks in advance for your feedback.