Good question. I don't have an answer, but I guess there could be tiers? Like, if a person* has a couple hundred karma, has been active on the site for a while, and has conducted him/herself well then that person could receive low level access. With the concern you brought up it's hard to choose criteria that would make a user trustworthy but that wouldn't warrant just letting them in completely. I guess I would advocate less stringent requirements. Like, nobody with negative karma and to be accepted you need to have been on the site for x amount of time and have been polite/non-inflammatory/thoughtful in all previous discussions. If a person has low karma because they rarely comment, they likely won't post much in the email list anyway.

If we need a way to find out if someone's trustworthy, can't we just ask them to raise their right hand?

*This hypothetical person happens to be me.