Bring acoustic instruments, or just bring your voice. The concept is simple—we get together and just play songs.
How it works: We basically just go around the circle and take turns; each person has the option of leading a song, requesting a song from someone else, or just passing it to the next person. If you want to bring copies of lyrics or charts for complex songs, you can, but it's not required. Sometimes it's nice to say something like "It's in the key of G!" The goal is to try to make a harmonious noise together, so be warned, there may be tuning involved.
Hope to see you there!