The NL-1331 Meetups medal is awarded for attending NL-1331 events. Check out the Ingress Live Events page to learn when and where NL-1331 events are occurring. Note that in order to get credit for attending, you must register on the official event page.
The NL-1331 medal was originally untiered, but in January 2018 Niantic converted it to a tiered medal. No retroactive credit was given for previous NL-1331 events attended.