TL;DR News Link pages are beta-only, so you need the Microsoft.Graph.Beta NuGet package. For banner image uploads, you also need Microsoft.Kiota.Serialization.Multipart. The tricky part is that the multipart case has no official C# snippet — Microsoft’s documentation just says “Snippet …

TL;DR Add WelcomeEmailDisabled to resourceBehaviorOptions via AdditionalData when creating the group. It can only be set at creation time — not patched afterward. AdditionalData = new Dictionary<string, object> { { "resourceBehaviorOptions", new List<string> { …