Privacy
When choosing your settings, you are asked to make your groups pages private or public. Private means that only members can see you calendar, members page, photos, messages, and polls. Non members can still see your welcome page (except for your whats new category), and your about page.
Advantages of a Public Group
A public group is a way to attract more members. By being able to see the calendar, potential members can see if it has the type of events they are looking for, and get to the know the members in the group, by lurking. This really helps shy members. Also, when an organizer is having a problem, is the group is public, they can post on the organizer forum and other organizers can easily see their groups to help diagnose the problem.
Disadvantages of a Public Group
With a public group, members are reluctant to share sensitive information like their kid's names, and ages, as well as phone numbers and their location, which can be crucial to organizing. also with information being attainable to anyone with Internet access, it's possible to attract not so nice people (like a sore ex husband of a member, or worse-- a sexual predator).
Advantages of a private group
In a private group, your members' information is only accessible to other members. With it feeling more safe members are more comfortable to get to know each other on the message board. Also, members that have to actually 'join' to be able to participate; this brings in more motivated members.
Disadvantages of a private group
By having pages private, no one outside of your group (besides meetup staff) can see inside your group. When you have a problem with software, you may have to wait for meetup staff to help you out. Also you may loose some pontentail members because they are unsure of what the group's like.