...People have all sorts of reasons for attending NA meetings, but the reason for each meeting is to give NA members a place to share recovery with other addicts. If you are not an addict, look for an open meeting, which welcomes nonaddicts. If you're an addict or think you might have a drug problem, we suggest a meeting every day for at least ninety days to get to know NA members and our program...from the Service Pamphlet, An Introduction to NA Meetings
NA meetings all have the same primary purpose: to carry the message of recovery to any addict that may need it. Meetings vary in length, format and focus areas, and may sometimes be aimed at specific interest groups - but all have the same primary purpose.
At NA meetings, we introduce ourselves as addicts and try not to focus on specific drugs, but rather the disease of addiction. The reasons for this are outlined in the clarity statement.
There is no requirement for attending an NA meeting. You do not need to book, or have come through any kind of facility. Many meetings are attended by addicts in rehabilitation centres, and some meetings may even be held at these centres. Some meetings are held in church facilities or community centres. NA has no affiliation to any of these, or any other venue.
An example of the format of a meeting can be found here (under sample meeting formats), and may give you an idea of the layout of a meeting. Meetings may have a topic, or an individual sharing his or her experience of recovery. For a guide to terminology used in meetings, see an introduction to NA meetings.