From the Wikipedia page 
The Grotius Society was a British society founded in 1915 during World War I. In 1958, it was dissolved on the merger with the Society of Comparative Legislation, founded in 1895, to form the British Institute of International and Comparative Law.
The society's objectives were "to afford facilities for discussion of the Laws of War and Peace, and for interchange of opinions regarding their operation, and to make suggestions for their reform, and generally to advance the study of international law."
Members had to be British subjects, but the society undertook work for the International Law Association.