We meet every Sunday, at approximately 7:30 PM. The next Beowulf Cluster will probably be located at Southern Pacific, located quasi-near 16th & Mission BART, but you should read the mailing list archives to know for sure!
Beowulf Cluster is attended by people who care about free software, and people they care about. You are welcome to join if you know someone else who attends the event. (We aim to keep it a relatively intimate social gathering; invites are distributed liberally, but are a good idea.)