We meet once a month to drink beer & share lessons learned on various topics including computer science, design, HCI, complex systems development, programming languages, compiler optimization, cryptography, computer music, AI... and whatever else the group finds interesting.
The common thread is that most of the presenters & audience know at least one dialect of lisp (eg Common Lisp, Scheme, Clojure, Emacs Lisp, etc). But it is definitely a polyglot (not just lisp-related) group & presentations on non lisp-related subjects are encouraged
The format is usually "educational lightning talks" (background + 5 minute presentation + 5 minutes for QA), with all attendees invited to present if they feel like it. Followed by free beer.