Session session1 = ...;
Session session2 = ...;

Club club = session1.get( Club.class, 1 );

Person p1 = session1.get( Person.class, 1 );
Person p2 = session2.get( Person.class, 1);

club.getMembers().add( p1 );
club.getMembers().add( p2 );

// this evaluates to ... well it depends
assert club.getMembers.size() == 1;