Ok, I haven't had a lot of time to think through all of this (been busy working on a few things here at work 😉), but here's my quick take:
1. If we restrict the email to the current ACTIVE members of TA, I don't see how that could be considered SPAM. If it was to inactive or non-team members, then it would be SPAM.
2. We don't need a long letter, just a few short sentences, STARTING with a hearty thank-you for participating in Team Anandtech's distributed processing efforts.
3. It should include a sentence about Team Anandtech's current and expected rankings.
4. It should also include an invite to check out the team website link, the Distributed Forums link, the Gamebox link, IRC channel, etc.
5. It should end with a way (maybe 2 links, one to join the TA newsletter, and one to not join and not receive any further mailings) for them to join the TA newsletter.
6. If we do not hear from them one way or another on the newsletter, we should assume they don't want to be contacted or added to the newsletter.
I agree, getting more existing members active on the forum and IRC and the gamebox and races, etc. can help improve Team Anandtech's keyrate!
😀