Was the exact opposite at the beginning of april (when I first started playing this mod). Almost nobody played in the NA server, most NA people were on an EU server. After a couple days of myself (and a few friends I knew from other warband mods) started advertising in the EU server to come join the NA server (and sticking it out on the NA server when it was only a handful of people on at all times) it seemed to get populated within a couple days. Slowly more and more started coming on.
It's gonna be a gradual process, but if you get people in the EU server and can keep them there, more will follow. Also, having admins on is a necessity...if they are not, people probably won't stick around. Good luck