DynamicTreeTableModel and ComparableTreeTableModel contain methods for serializing the TreeTableComparators, and thus the group state, to an ObjectOutputStream:

public void saveComparators(ObjectOutputStream out);

An ObjectInputStream can also be used to load the TreeTableComparators:

public void loadComparators(ObjectInputStream in);

Note: In your application, you could write the group state to a file when exiting and load it upon initialization.