In this excellent answer to a related question, Christian notes that practicing lawyers are unlikely to use this site. The other potential pools of legal experts would be law professors and students, and retired practitioners. Perhaps government employees (law leforcement officers, state attorneys, legal clerks, and judges) and paralegals will have time and interest in sharing their expertise?
What can we do (and, as important, avoid doing) to attract and retain members of any of these pools?
Most lawyers, or at least most good lawyers, in my experience, are interested in the law, and like discussing it with other lawyers. The same goes for discussions with non-lawyers, under some circumstances.
The things that annoy lawyers about discussions with non-lawyers are more or less the things I've heard doctors complain about. Doctors don't want to talk to people who are just pumping them for free advice, and they don't want to talk to people who think they know better than all of modern medicine because they saw Jenny McCarthy on a talk show once.
I don't know about attracting lawyers, but the best way, in my opinion, to keep from driving lawyers away from the site is to avoid these two situations:
Lawyers won't post if they think they're going to face professional discipline for posting. The more questions that deal with legal theory and legal generalities, and the fewer that are outright, or thinly disguised, requests for advice for a poster's own specific legal difficulties, the more likely a lawyer is going to want to answer it (for free).
Lawyers have to deal, all too often, with the nutty fringe-on-the-flag type of legal "truther," who wants to explain that he's not subject to the court's jurisdiction because the summons used capital letters. If the questioners, voters, and commenters are hostile to real-life answers and the legal system, lawyers will take a pass.
These are things that will depend partly on policy, and partly on the user community.
It might not hurt to prime the pump by promoting the site (now that it's public) in places where lawyers and law students are likely to see it--Above the Law, for instance. A critical mass of people who know what they're talking about is probably the best way to avoid this type of problem.
You might not like this answer, but as Christian said in a related answer...
Almost all of the questions on this site are in the form of "what does the law say about issue x" (e.g. "Is it legal for me to jaywalk in [jurisdiction x]"). These questions are very useful to non-lawyers, who want to learn what the law says they can and can't do. However, lawyers spend very little time answering basic general legal questions. Most of their time is spent doing things like this:
Add in the fact that lawyers usually charge for their advice (and the fact that giving legal advice can have legal consequences), and I can't see why legal professionals would want to participate in a site that doesn't have much to do with what legal professionals actually do.
(You got this advice before the proposal began, so this should be nothing new.)
I don't really know how this issue can be fixed. Most (almost all) of the people who committed to this proposal are not legal professionals, so if a legal professional were to come and ask an "advanced" question, it probably would not be well received. And law isn't like programming: it's very hard to learn to be a lawyer through the internet, so I doubt that any users here will learn to be lawyers as they use the site.
This site can still be valuable, but it will have to find a role to fill that doesn't require legal experts.
I wonder if moderators here have mulled contacting or advertising moderators of law Sub-Reddits like r/ask_lawyers, r/auslaw, r/lawyers ("private subreddit for licensed attorneys"), r/lawcanada, or r/uklaw?
The main thing is just making sure this site steers well clear of situations where an AC relationship could be established, which means sticking to actual hypotheticals, and allowing law students and professors to add personal disclaimers in their posts. Law students like me love tricky hypotheticals, and practicing explaining legal answers in plain english (rather than legalese) is great.
