@ManyToMany(
            cascade={CascadeType.PERSIST, CascadeType.MERGE},
            mappedBy="users",
            targetEntity=Service.class
    )   
    private Set<Service> service;
    public Set<Service> getService() {
        return service;
}

@ManyToMany (
            targetEntity = User.class,
            cascade = {CascadeType.PERSIST, CascadeType.MERGE}
    )
@JoinTable(
            name = "SERVICE_USERS",
            joinColumns = @JoinColumn ( name="SERVICE_ID" ),
            inverseJoinColumns = @JoinColumn(name = "USER_ID")
    )
private Set<User> users;
public Set<User> getUsers() {
        return users;
}

Service service = new Service();
      
service.setTitle("Delete folders");
service.setDescription("Desription of service");
service.setDate( new Date() );
service.setCount( 0 );

User user = new User();
user.setFio("Ivanov Sergei Ivanovich");
user.setCreateDate(new Date());
user.setUpdateDate(new Date());
Set <User> set =  new HashSet <User>();
set.add(user);
service.setUsers(set);

ORMHelper.update(service); // error

Hibernate: /* insert ru.mipt.rivis.hib.example.Service */ insert into Service (COUNT, DESCRIPTION, LASTCALLDATE, TITLE) values (?, ?, ?, ?)
Hibernate: /* insert collection row ru.mipt.rivis.hib.example.Service.users */ insert into SERVICE_USERS (SERVICE_ID, USER_ID) values (?, ?)
org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: ru.mipt.rivis.hib.example.User