How would any organizer reach adjuncts to tell them there was a meeting? This is a huge urban area, so an itty bitty ad in the newspaper isn't likely to grab much attention. Forget flyers or posters on campus. They would be confiscated immediately.
May be illegal: http://www.nlrb.gov/faq
Whether it is illegal or not is hardly relevant. The administration is likely to do it anyways, if they feel an organizing drive might be taking off, and confiscating the posters might slow it down; if you're involved in such a situation, documenting it can be useful, as a record of unfair labor practices can be a bargaining chip later on when negotiating various things during an organizing drive. As to how to reach adjuncts, you have to be creative - course schedules and word of mouth probably. It might also be possible to get department adjunct lists from sympathetic administrators and faculty. It could be a bit tricky knowing how large the bargaining unit will be, to be sure you've got enough cards signed.
Forming a committee means locating a core of activists willing to do really tough work, and to risk losing their jobs (also illegal, but also a common tactic). The only way to do that is one-by-one; you don’t start with a big meeting – you start with a small meeting of a few people you know and trust and ask them to help you locate these activists through their own contacts (not all, or maybe not any, of your initial group will be the ones to actually be on the committee in the end, as it takes a certain kind of committed person to persist). If you contact a union, they might be able to send an organizer to help with the work, but they can only do a small bit, and help with the strategy development, perhaps provide some resources, but most of the work has to come from the committee. It is hard, tedious work, and many of the people you want to help will not appreciate it, so this is why most people in the US don’t have union representation, although any rational person with full knowledge of the implications would want it if given the choice.