@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

private String groupName;

@Transient
private Long user_id;

private String description;

@OneToMany(mappedBy = "group",fetch = FetchType.LAZY)
private Collection<Task> tasks = new ArrayList<>();

@ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
@JoinTable(
        name = "user_group",
        joinColumns = @JoinColumn(name = "group_id"),
        inverseJoinColumns = @JoinColumn(name = "user_id"))
private Collection<User> users = new ArrayList<>();

@ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
@JoinTable(
        name = "admin_group",
        joinColumns = @JoinColumn(name = "group_id"),
        inverseJoinColumns = @JoinColumn(name = "user_id"))
private Collection<User> admins = new ArrayList<>();
...

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

@NotNull
@Size(min = 3)
private String userName;

@Email
@NotNull
private String email;

@NotNull
@Size(min = 3)
private String firstName;

@NotNull
@Size(min = 3)
private String lastName;

@NotNull
@Size(min = 5)
private String password;

@ManyToMany(mappedBy = "users",fetch = FetchType.LAZY)
private Collection<Group> groups  = new ArrayList<>();
...