If you are a Psychology student for the entire bachelor:
You can easily become a member by filling out this online registration form. If you don't have a Dutch IBAN number, you can create an account with Sofort.
If your IBAN number isn't supported by iDeal (for Dutch IBAN numbers) or Sofort (for most international bank accounts), you can fill in the physical registration form and send it to firstname.lastname@example.org, or stop by the Labyrint common room to fill out a form (SA55).
If you study Psychology here for only half a year:
Fill out the registration form in the Labyrint common room (SA55) and pay by card right away. Depending on your membership you pay either €9,- for a semester or €17,50 for a year*.
*You are required to pay the membership fee that is specified in the House Rules.