Ask HN: Suggestions about "remote" sharing startups

Hi. I am a freelance web developer. Last week I started working with a guy who is looking to build the core team for his startup (initially he just need a tech man who develop the app from scratch). We have good connection, so I joined to the project as the CTO. I am charging for this project the third part of I usually charge, but he also offer me the 15% of sharing, so I finish agreeing.

The main point with that is our locations, he is based on Netherlands and I live in Argentina. And the business will be focused on Europe.

So, my question are, what should I know about "remote sharing"? Is it legal? what papers do I have to request him?

Everything you can suggest me would be appreciated!

thanks