Perhaps the better questions would be: Why both a Meetup site and this Joomla website? Why not just one or the other? Why make me register in two different places?
Meetup.com provides the group with several advantages. It comes with a pre-built audience which includes quite a number of local web developers and internet entrepreneurs. Individuals on meetup are notified whenever a new group starts that may be similar to their interests. Meetup groups also rank high in search engines. By being a part of Meetup.com, we increase our visibility. The event calendar on Meetup also allows for automatic notifications of members when meetings are created or changed as well as reminders about those meetings. So, being a part of Meetup.com also increases our meeting attendance.
So why bother with a website? Why not just use Meetup for the user group? Frankly, the idea of a Joomla! User Group without its own Joomla! website would seem odd, to say the least. Plus, the website gives our members a wider range of methods for participation and for information.