Your domain model is wrong because a commissie has probaly multiple vrijwilligers and a vrijwilliger can be part of multiple commissions. So change the reference to multiple and set the option that both refer to each other. Now to view all the Vrijwilligers of a commission create a dataview below the grid that listens to this grid. In the dataview place a grid of vrijwilligers inside the dataview and let it fill over the reference. This way when you select a commissie it will show all the vrijwilligers.
Instead of using the dataview widget, try the list view widget.
Something like this:
Switch the relationship the other way around and then try