The Combinatorics Study Group (CSG) 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 Queeen Mary and from other London Colleges, and the discussion group moved to various rooms in college before finally settling in the Maths Seminar Room (M103). Peter continued to run the CSG until he retired from Queen Mary in 2013.
The ethos of the CSG 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 CSG 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 on Peter's web pages at http://www.maths.qmul.ac.uk/~pjc/csgold.html
The CSG is currently jointly organised by David Ellis and Robert Johnson. A list of forthcoming seminars can be found at http://www.maths.qmul.ac.uk/seminar-series/combinatorics-study-group