At last month’s service, more than 75 people filled the pews, including the family of Christopher Honor, who was Courtney’s boyfriend. He was also addicted to heroin. Last month, almost a year after her death, Chris, 22, died of an overdose — the 23rd overdose and third fatal one this year in Plaistow, a town of 8,000 people.

Chris’s mother, Amanda Jordan, 40, wanted to attend the Sunday night service last month, but it was just two weeks after she had buried Chris, and she worried it might be too soon to go back to that church, where Chris’s funeral was held. She sometimes thinks Chris is still alive, and at his funeral she was convinced he was still breathing.

She was afraid she would fall apart, but she and other family members decided to go anyway. During the service, her son Brett, 18, became so overwhelmed with emotion that he had to leave, rushing down the center aisle for the outside. Ms. Jordan ran after him. Then a family friend, Shane Manning, ran after both of them. Outside, they all clutched one another and sobbed.

“I’m a mess,” Ms. Jordan said after coming back inside and kneeling in front of a picture of Chris. In addition to yearning for her son, she had been worried that the Griffins blamed her for Courtney’s death. But at the church, they welcomed her. In their shared pain, the families spoke and embraced.

Ms. Jordan, one of the more recent involuntary members of this club of shattered parents, said that someday, when she is better able to function, she “absolutely” wants to work with the Griffins to “help New Hampshire realize there’s a huge problem.” Right now, though, she just wants to hunt down the person who sold Chris his fatal dose. “These dealers aren’t just selling it,” she said. “They’re murdering people.”