Highlander Magic Europe: https://www.facebook.com/groups/HighlanderEurope/
Magic Highlander: https://www.facebook.com/groups/magichighlander/
Tournament Series:
Metagame Master: https://www.facebook.com/groups/metagamemasters/
The One - Mannheim Highlander Series: https://www.facebook.com/groups/252578405122925/
Library of Slovakia (by iHRYsko) - Slovakian Highlander Series: https://www.facebook.com/groups/1652790018384381/
Local Groups:
Highlander in Berlin: https://www.facebook.com/groups/251169021562505/
Highlander in Mannheim: https://www.facebook.com/groups/111450495596635/
Other:
Canadian Highlander: https://www.facebook.com/groups/288447914624649/
feel free to add...