I have been looking into ABM and have a question: we can add people to Accounts based on their IP Address. Is this also valid for an anonymous visitor?
You can't really do anything with anonymous visitors except track their behaviour in munchkin and have the last three months associated with the person when they become known. Once someone becomes known you can use their munchkin data to perform actions.
on the Marketo ABM documentation I found this though:
So, while I can´t send them directly emails I still can know that the visitor from a Company X has visited my website. Correct?
It's quite limited what you can do in terms of triggers based on anonymous activity, but yes you'll be able to know if an anonymous person from a target account has visited your site.