public class ListOfUsers
{
    public Guid idUser { get; set; }
    public string FIO { get; set; }
}

public ICollection<ListOfUsers> First { get; set; }
public IEnumerable<Guid> Second { get; set; }
public ICollection<ListOfUsers> Result { get; set; }

foreach (var second in Second)
{
    foreach (var first in First)
    {
        if (second != first.idUser)
        {
            Ressult.Add(new ListOfUsers
            {
                idUser = first.idUser,
                FIO = first.FIO
            });
        }
    }
}

var first = new List<ListOfUsers>
{
    new ListOfUsers { idUser = Guid.Parse("B7AADF05-1F63-4A20-94B1-9A31F4AB4910"), FIO = "Ivan Ivanov"},
    new ListOfUsers { idUser = Guid.Parse("B7AADF05-1F63-4A20-94B1-9A31F4AB4911"), FIO = "Petr Petrov"},
    new ListOfUsers { idUser = Guid.Parse("B7AADF05-1F63-4A20-94B1-9A31F4AB4912"), FIO = "A K"},
    new ListOfUsers { idUser = Guid.Parse("B7AADF05-1F63-4A20-94B1-9A31F4AB4913"), FIO = "Sanvirtus Sanvirtus"},
};

var second = new List<Guid>
{
    Guid.Parse("B7AADF05-1F63-4A20-94B1-9A31F4AB4910"),
    Guid.Parse("B7AADF05-1F63-4A20-94B1-9A31F4AB4911"),
};

var result = new List<ListOfUsers>();

foreach (var element in first)
{
    var isElementPresentInSecond = second.Any(x => x == element.idUser);

    if(isElementPresentInSecond)
    {
        continue;
    }
    else
    {
        result.Add(element);
    }
}

class ListOfUsersByIdEqualityCmparer : IEqualityComparer<ListOfUsers>
{
    public bool Equals(ListOfUsers x, ListOfUsers y)
    {
        return x?.IdUser == y?.IdUser;
    }

    public int GetHashCode(ListOfUsers obj)
    {
        return obj?.IdUser.GetHashCode() ?? 0;
    }
}

var result = new HashSet(first, new ListOfUsersByIdEqualityCmparer());
result.ExceptWith(second.Select(s=>new ListOfUsers(){ idUser = s }));

var result = first.Except(second.Select(s => new ListOfUsers() { IdUser = s }), new ListOfUsersByIdEqualityCmparer());