Send email as calendar invite/appointment in SendGrid C#

According to your description, I checked this issue and tried to send a email with the calendar attachment. You could refer to the following code snippet:

static async Task SendGridAsync()
{
    var client = new SendGridClient("your-api-key");

    var msg = new SendGridMessage()
    {
        From = new EmailAddress("{sender-email}", "{sender-name}"),
        Subject = "Hello World from the SendGrid CSharp SDK!",
        HtmlContent = "<strong>Hello, Email using HTML!</strong>"
    };
    var recipients = new List<EmailAddress>
    {
        new EmailAddress("{recipient-email}", "{recipient-name}")
    };
    msg.AddTos(recipients);

    string CalendarContent = MeetingRequestString("{ORGANIZER}", new List<string>() { "{ATTENDEE}" },"{subject}","{description}", "{location}", DateTime.Now, DateTime.Now.AddDays(2));
    byte[] calendarBytes = Encoding.UTF8.GetBytes(CalendarContent.ToString());
    SendGrid.Helpers.Mail.Attachment calendarAttachment = new SendGrid.Helpers.Mail.Attachment();
    calendarAttachment.Filename = "invite.ics";
    //the Base64 encoded content of the attachment.
    calendarAttachment.Content = Convert.ToBase64String(calendarBytes);
    calendarAttachment.Type = "text/calendar";
    msg.Attachments = new List<SendGrid.Helpers.Mail.Attachment>() { calendarAttachment };

    var response = await client.SendEmailAsync(msg);
}


private static string MeetingRequestString(string from, List<string> toUsers, string subject, string desc, string location, DateTime startTime, DateTime endTime, int? eventID = null, bool isCancel = false)
{
    StringBuilder str = new StringBuilder();

    str.AppendLine("BEGIN:VCALENDAR");
    str.AppendLine("PRODID:-//Microsoft Corporation//Outlook 12.0 MIMEDIR//EN");
    str.AppendLine("VERSION:2.0");
    str.AppendLine(string.Format("METHOD:{0}", (isCancel ? "CANCEL" : "REQUEST")));
    str.AppendLine("BEGIN:VEVENT");

    str.AppendLine(string.Format("DTSTART:{0:yyyyMMddTHHmmssZ}", startTime.ToUniversalTime()));
    str.AppendLine(string.Format("DTSTAMP:{0:yyyyMMddTHHmmss}", DateTime.Now));
    str.AppendLine(string.Format("DTEND:{0:yyyyMMddTHHmmssZ}", endTime.ToUniversalTime()));
    str.AppendLine(string.Format("LOCATION: {0}", location));
    str.AppendLine(string.Format("UID:{0}", (eventID.HasValue ? "blablabla" + eventID : Guid.NewGuid().ToString())));
    str.AppendLine(string.Format("DESCRIPTION:{0}", desc.Replace("\n", "<br>")));
    str.AppendLine(string.Format("X-ALT-DESC;FMTTYPE=text/html:{0}", desc.Replace("\n", "<br>")));
    str.AppendLine(string.Format("SUMMARY:{0}", subject));

    str.AppendLine(string.Format("ORGANIZER;CN=\"{0}\":MAILTO:{1}", from, from));
    str.AppendLine(string.Format("ATTENDEE;CN=\"{0}\";RSVP=TRUE:mailto:{1}", string.Join(",", toUsers), string.Join(",", toUsers)));

    str.AppendLine("BEGIN:VALARM");
    str.AppendLine("TRIGGER:-PT15M");
    str.AppendLine("ACTION:DISPLAY");
    str.AppendLine("DESCRIPTION:Reminder");
    str.AppendLine("END:VALARM");
    str.AppendLine("END:VEVENT");
    str.AppendLine("END:VCALENDAR");

    return str.ToString();
}

Result: