Person one = new Person("one");
Person two = new Person("two");
Person three = new Person("three ");
Person four = new Person("four");
PersonGroup group = new PersonGroup();
group.add(one);
group.add(two);
group.add(three);
group.add(four);

// Persisting group
em.persist(group);

Pair pair = new Pair(one, three);
em.persist(pair);