The Combinatorics Study Group has been running since 1986.
It began as a discussion group in Peter Cameron’s office between Peter and two PhD students, Mark Whelan and Liam Halpenny. Other people soon joined in, both from Queen Mary and from other London universities. Peter continued to run the group until he retired from Queen Mary in 2013.
The ethos of the Combinatorics Study Group has always been on interactive presentations with speakers encouraged to present work in progress and open problems, and participants strongly encouraged to take part in discussions during the presentations. Meetings of the group regularly have 20-30 participants from a wide range of backgrounds.
There have been several notable series of talks introducing active areas of research such as those given by Rafael Sorkin on causal sets and Alan Sokal on the multivariate Tutte polynomial. A list of meetings going back to 2000 can be found here.A list of forthcoming seminars can be found here.