I have no connection to Meetup, but I think it provides more than a mailing list, namely:
* a flow of new people who find the events geographically and topically
* member data so you know who you’re meeting, tied to attendance
* picture galleries
* an easy way to track many meetings you attend per month, instead of each group needing its own system
* automatic email reminders
* suggestions and voting on venues
* an ongoing discussion board tied to the service
* and, dare I hope, more cool features as the service starts earning money