It's a dead link Desi C. If you're in the UK there's the national convention near Sheffield in August. My experience is everyone is made welcome, just bring some food to share for lunch and be prepared to sing your heart out. Although Sacred Harp is rooted in a particular cultural and religious framework nobody is going to pick your brains on the subject and people of various faiths and none attend singings.
You pick it up soon enough and nobody is criticised for singing a wrong note. It's that rare thing, a relevant, inviting, participatory art form in the modern world. Plus you get to eat like a horse.
Sacred harp Convention