In Europe, two official meetups take place during Pokémon GO Wild Area: Global, organized with local Community Ambassadors:
Karlsruhe (Germany): Saturday, November 14, from 10:00 a.m. to 8:00 p.m. local time, at Ettlinger Tor (Karl-Friedrich-Straße 26, 76133 Karlsruhe).
Stockholm (Sweden): Sunday, November 15, from 10:00 a.m. to 6:00 p.m. local time, at Kulturhuset Stadsteatern (Sergels Torg, 111 57 Stockholm).
At the venue: Timed Research, only for Trainers who are there and check in at the official meetup. You’ll also find activities with local communities, photo spots, and Pokémon GO pins and prints (while supplies last). Extra PokéStops, Gyms, and Powerspots are added near the venues. You can also meet HanniMew in Karlsruhe.
What to do: RSVP with the links in the official announcement. The Campfire groups of the participating communities (Karlsruhe, Stuttgart, Pforzheim, Vaihingen an der Enz, Stockholm) share the details.
Not near these cities? Other communities are planning meetups all over Europe: find your local community.
All the details in the official announcement.