Under US law, you can as long as you meet the US tax & legal obligations:
- You have to meet the accredited investor requirements.
- You need to wire the money from a bank that has appropriate legal controls (any bank not in one of the countries on this list).
We do not market to European investors, so you should check with your local lawyer about whether you can get involved. Investors outside (or inside) the U.S. should also get good tax advice when investing in startups.