private async Task <String[]> GetPropertyOwnersAsync(Graph.GraphServiceClient graphService)
{
var resturl = "/_api/lists/GetByTitle('Properties')/Items?$select=sl_owner";
var responseString = await RestHelper.GetDemoSiteJsonAsync(resturl, _token);
var properties = JObject.Parse(responseString)["d"]["results"].ToObject <Property[]>();
var users = await graphService.GetAllUsersAsync(properties.Select(i => i.sl_owner));
return(users.Select(i => i.Mail).ToArray());
}