"how to efficiently spam a group that is of a very large size, say about 100,000 users"
I'm guessing that's what Chris reacted to. It's certainly what I reacted to.
If you are genuinely talking about sending messages to a large group of people who have explicitly asked for them, and who can opt-out at any time, then you are talking about a mailing list. There are plenty of resources about to give you some ideas on how to do this.
Spamming is a completely different matter. It's force-feeding messages to people who would not otherwise have requested them.
Discussion groups like this one can be very helpful. People give their time and advice for free. It's hardly surprising that some of us react negatively when someone asks for help to do something as netnasty as spamming.
If you'd like some help on what spam is and isn't, and why it annoys so many of us, take a look at http://spam.abuse.net
Regards
Jack