LEAD Ownership

Is there a way that you can MASS UPDATE the Lead Owner Field? I am importing from a CSV thousands of leads and I would like to assign them in bulk to certain saile people. Is there a way to either MASS UPDATE or even better, how do I know what LEAD OWNER ID to enter for a specific person? Is it a number?

I tried thier names and that does not work, it seems to ignore it and still assign me as the owner since I imported those leads.