Sites like Northeastshooters.com and Glocktalk.com aren’t private; anyone can read the forums. You only need an account if you’re going to post a thread or reply to one. So, why would anyone (like Kaynbred in multiple cases) register accounts with all of these sites and then never post?
Well, registering a forum account does afford a few other benefits. Namely, it allows you to view the profiles of other users, and to send them private messages.
I don’t have any indication that Adam Lanza did send any messages to other users (probably the only people that would know would be the site admins and the users in question), but it’s certainly possible, especially given his admission that he “prefers to use a proxy whenever possible” when asking about firearms information.