So evidently up to now it has not been illegal to belong to these groups. From Expatica, with thanks to Anthony:
AMSTERDAM “” The Netherlands plans to ban all terrorist groups listed on the European Union’s blacklist.
Active involvement with such groups will be a criminal offence, the Dutch government said on Tuesday.
The groups on the blacklist include the PKK (Kurdish Workers Party, now called Kongra-Gel), the Palestinian group Hamas, the Islamicist organisation Al-Takfir, the Muslim Al-Aqsa Nederland foundation, and the Marxist New Peoples Army (NPA) of the Philippines.