Here are the official guidelines for posting on the CocoaHeadsNYC mailing list. (Google Groups doesn't seem to provide a good place to put this information, hence this page.) Thanks to all who have posted input on this matter.
Two things are definitely on-topic for this list:
Discussion about the meetings themselves. Announcements, logistics, recaps, topic suggestions, followup questions.
Technical Q&A. In practice, most of us go first to other resources like Stack Overflow and cocoa-dev. But tech questions are welcomed and on-topic.
Other topics are allowed but in various degrees of gray:
Meta-discussion about the list and the group. On cocoa-dev, "admin" threads are picked off fairly quickly and are considered off-topic. However, because we're still a small, face-to-face group, it's fine to start such discussions on this list. I'll keep an eye out for dead horses, going in circles, or things getting personal, at which point I'll either kill the thread or ask that it be taken offline.
Requests for feedback on our own apps. I can see how these can come off as ad-like or spammy, but opening ourselves up for critique is actually something I'd like to encourage in emails, presentations, and the freeform conversation time at our meetings. For now I'll let these requests stand. Please try not to be spammy.
Job postings and requests for beta testers. I lump these together because I think they're the most clearly off-topic. Rather than ban them outright, the rule is that job postings must have "[JOB]" in the subject line and requests for testers must have "[BETA]" in the subject line. Thus, members who are interested in such posts can spot them more easily, and members who are not interested can filter them.
Other things to know:
First-time posters will be placed on moderation.
Spammers with no other posts will be removed from the list.
When in doubt, bear in mind that the purpose of the group is to help each other become better Cocoa and iOS programmers. That's why we have presentations, demos, help-me-with-this-bug sessions, and hopefully please-critique-my-app discussions. These are ways in which our proximity helps us help each other — hence the "NYC" in "CocoaHeadsNYC". The socializing is a natural and happy consequence of everything else.