For Artificial Intelligence, there is Axon. Axon was founded in 2005, back when the study was called "Technische Cognitiewetenschappen" (Technical Cognitive Science). To join Axon and to become an alumnus you must have a Bachelor's or Master's degree in Artificial Intelligence. They organise drinks for their members every other month. Membership for Axon is €5 per year.
More information about Axon can be found at their website. Note: Their website is only available in Dutch.
Invariant - Computer Science
For all Computing Science graduates, there is the alumni association Invariant. Invariant was started in 2013 and is open to all Bachelor and Master graduates of Computing Science. The membership fee for Invariant is €10 per year.