where people are already located

The wiki list of European meetup groups, filtered by "at least once in a month" (mentioned explicitly in the group description), gives us the list of possible candidates:

• Vienna, Austria
• Brussels, Belgium
• Prague, Czech Republic
• Berlin, Germany
• Hamburg, Germany
• Munich, Germany
• Kraków, Poland
• Poznan, Poland
• Moscow, Russia
• St. Petersburg, Russia
• London, UK

Note that some of the information may be out of date, the wiki page does not mention the average number of people attending the meetups, etc. (Also, it's not just the number of members that matter, but how seriously they take the topic, etc.) So we can use this as an "upper bound" on places where enough active European LW-ers live.

As the next step I would recommend looking at what the local groups have already done, using the "past behavior is the best predictor of future behavior" heuristic. Berlin seems like an obvious winner; they organized the 2014 and 2015 European LW Community Weekends.