study-lib-backend/API/Models/User.cs

using Microsoft.AspNetCore.Identity;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;

namespace StudyLib.API.Models
{
    public class User : IdentityUser
    { 
        public string FullName { get; set; }

        public ICollection<Group> Groups { get; set; }
    }
}