The phone number for Yola's Foster Road Coffeehouse in Portland, OR is 503-774-4516. The sunday session is an instrumental jam OR a song circle, depending on who shows up. No real rules.
Also, while the Portland Folklore Society link has been posted above, I'd like to emphasize the locations for song circles.
PFS 2nd  SATURDAY SONG CIRCLE -- 3738 SE Lincoln; 503-231-1387

Monday nights thru Oct. 29,  2001 SONG CIRCLE - 7:30-9:30PM, 941 SE Bidwell in Sellwood; Info: 503-232-3835.

2ND WED ACOUSTIC JAM - 7 - 9 PM, 503-238-1270