To make a political party, you need to be an organisation with at least 2 members. Well, the PVV gets around this by having Wilders as a member, and another organisation as the second member, which is also Wilders, but as another legal entity.
But... for voting people make a list. This one is separate from the organisation or party, so it can also include people who are not members of the party. Basically, it is an agreement between these people that if Wilders get more votes then needed to get him into parliament, the remaining votes will pass down the list until they run out.
Those people can actually do whatever they want when in parliament. They can split off, vote differently, etc. But if they do, of course Wilders won't want them on the list (which he decides) the next time.