public struct Meeting
{
    public string Id { get; set; }

    public DateTime Start { get; set; }

    public DateTime End { get; set; }
}

public static IEnumerable<ILookup<D, T>> GroupBy<T, D> (this IEnumerable<T> outer,
                                                        Func<T, IEnumerable<D>> func)
{
        if (outer == null)
            throw new ArgumentNullException(nameof(outer));

        if (func == null)
            throw new ArgumentNullException(nameof(func));

        Dictionary<D, IEnumerable<T>> lookup = new Dictionary<D, IEnumerable<T>>();

        foreach(T item in outer)
        {
            IEnumerable<D> keySelectory = func(item);
            if (keySelectory != null)
            {
                foreach(D newKey in keySelectory)
                {
                    if (!lookup.ContainsKey(newKey))
                    {
                        lookup.Add(newKey, new List<T>() { item });
                    }
                    else
                    {
                        List<T> temp = lookup[newKey].ToList();
                        temp.Add(item);
                        lookup[newKey] = temp;
                    }
                }
            }
        }

        return WHAT?
}

meetings.GroupBy(meeting => GetDatesFromStartAndEndTimes(meeting));