ESN the Netherlands has 17 sections which are all run by their own board which is often supported by additional volunteers (via for example committees, teams, clubs, and/or buddy/mentor programs). You can gain information about joining any of these boards or become a supporting volunteer by sending an email to the respective section. You can find the websites of the local sections here. On their website you will find the email addresses of those people you need to contact when you want to become active on the local level.
If there is no local section yet in your city, you can always start a section or apply for a preexisting initiative to become a section. More information on becoming a section can be found here.