Within the offices of the Passim Center on Church Street, the ’60s folk music revival is alive and well. Photographs of Joan Baez crowd the walls. A framed, unpublished Bob Dylan poem memorializes a local, late-night writing frenzy. Jim Field from the ’60s bluegrass band Charles River Valley Boys stops in to “drop off his bags” before taking a walk around Harvard Square. And Betsy Siggins, the 69-year-old artistic director, keeps a sign in her entranceway that reads: “Hippies use side door.”