For me, the meetups that have been the most valuable to me have been the kind where there's a small crowd of people (50-150), and either an individual is being interviewed, or there's a panel discussion, followed by a question & answer session where anybody in the audience can have a good opportunity to ask questions to the person or people on stage and have a chance to be pointed in the right direction.
Usually these types of meetups are filled with people covering the whole range of experience, from no experience and just curious, all the way through to professionals working in the field, and having that range of experience leads to easy discussion, and lots of excitement as people begin to understand something new!