Create classes from pojo like that then parse to retrofit Convert json response to classes
    public class MyPojo{
    private User user;
    public User getUser ()
    {
        return user;
    }
    public void setUser (User user)
    {
        this.user = user;
    }
}
User Class:
    public class User{
    private String username;
    private String updated_at;
    private String _id;
    private String access_level;
    private String email;
    private String token;
    private String userid;
    private String __v;
    private String created_at;
    private Groups[] groups;
    public String getUsername ()
    {
        return username;
    }
    public void setUsername (String username)
    {
        this.username = username;
    }
    public String getUpdated_at ()
    {
        return updated_at;
    }
    public void setUpdated_at (String updated_at)
    {
        this.updated_at = updated_at;
    }
    public String get_id ()
    {
        return _id;
    }
    public void set_id (String _id)
    {
        this._id = _id;
    }
    public String getAccess_level ()
    {
        return access_level;
    }
    public void setAccess_level (String access_level)
    {
        this.access_level = access_level;
    }
    public String getEmail ()
    {
        return email;
    }
    public void setEmail (String email)
    {
        this.email = email;
    }
    public String getToken ()
    {
        return token;
    }
    public void setToken (String token)
    {
        this.token = token;
    }
    public String getUserid ()
    {
        return userid;
    }
    public void setUserid (String userid)
    {
        this.userid = userid;
    }
    public String get__v ()
    {
        return __v;
    }
    public void set__v (String __v)
    {
        this.__v = __v;
    }
    public String getCreated_at ()
    {
        return created_at;
    }
    public void setCreated_at (String created_at)
    {
        this.created_at = created_at;
    }
    public Groups[] getGroups ()
    {
        return groups;
    }
    public void setGroups (Groups[] groups)
    {
        this.groups = groups;
    }
}
Groups Class:
public class Groups{
      private String id;
private String _id;
private String name;
public String getId ()
{
    return id;
}
public void setId (String id)
{
    this.id = id;
}
public String get_id ()
{
    return _id;
}
public void set_id (String _id)
{
    this._id = _id;
}
public String getName ()
{
    return name;
}
public void setName (String name)
{
    this.name = name;
}}
Now follow that put your classes and get desire response Retrofit Json response