Game Theory Society: gametheorysociety.org
COST: http://www.cost.eu
The Mathematical Optimization Society: http://www.mathopt.org
The Association of European Operational Research Societies: https://www.euro-online.org/web/pages/1/home
GDRJeux: https://sites.google.com/site/gdrjeux/
COCO : https://www.maastrichtuniversity.nl/research/conflict-and-cooperation
The Game-Theory Research Theme at Maastricht University.