switching.social
If you want to use a free open alternative to MeetUp right now, there's GetTogether:
It's currently got a lot of IT-related groups, but it can be used for any topic.
You can follow GetTogether on here:
If you want to help with its development, the source code is here:
Bobinas P4G is a social network. It runs on GNU social, version 2.0.1-beta0, available under the GNU Affero General Public License.
All Bobinas P4G content and data are available under the Creative Commons Attribution 3.0 license.