Requirements to work in Spain (US citizen) (Madrid)

Hello everyone,

I'd like to ask for your help! A friend of mine (from the US) would like to relocate to Madrid and we are not sure what the requirements are.

The company has an office both in New York and Madrid, but they're not sure about the requirements to bring my friend to work in here. Could any US expat help me please?
We asked the US embassy, but they're not vey helpful!

Where can we go to get information? has anyone gone thorugh the same situation and can advice us?

Any suggestions are welcome. Thanks so much!