Manuel and Patricia Oliver had already been on the road for more than a week when they pulled their school bus bearing an American flag into a city park in Uvalde, Texas. They were unsure of just how many people would greet them on that sweltering day.

Then the families started arriving. Parents, grandparents, siblings and other kin of some of the 22 people killed last year at Robb Elementary streamed into the park, embracing the Olivers and each other. So, too, did a woman who lost her daughter at a school shooting in Santa Fe, Texas, where 10 people were killed in 2018. The Olivers had driven halfway across the country to Uvalde with their own story: The couple’s son, Joaquin, was one of the 17 people killed at Marjory Stoneman Douglas High School in Parkland, Fla., five years ago.

The Olivers came to Texas one day last month to find others who also understand what their lives had become and to work with them to prevent such tragedies from happening again.

“I’m looking to help and also to receive help,” Mr. Oliver said. “We all know that we exist. What if we start planning together? What if we can support each other?”

As mass shootings continue to erupt through schools, malls and entertainment venues across the country, a growing league of families have found themselves bound to one another by unfathomable grief. In late-night phone calls and in-person gatherings, they have shared advice and tears with other parents from shootings past, knowing that no one else could understand what it means to lose a child in a method so violent, and so public.

Rhonda Hart, the mother from Santa Fe, came to the Uvalde event last month because some of the family members there are now among her closest friends. They are all part of “the worst club imaginable,” she said.

The Olivers set off this summer to travel the country in a retrofitted school bus, stopping to remember victims in two dozen places that have attained a painful notoriety. Among them: Littleton, Colo. (13 killed in 1999); Aurora, Colo. (12 killed in 2012); Charleston, S.C. (nine killed in 2015); Orlando, Fla. (49 killed in 2016); Las Vegas (58 killed in 2017) and Nashville (six killed this year).

In the coming weeks, their stops will include visits to Newtown, Conn. (26 killed in 2012), then the United Nations in New York and the Capitol in Washington. The effort is funded by the couple’s nonprofit, Change the Ref.

As they arrived in Uvalde, with temperatures approaching 100 degrees, Ms. Oliver said they were motivated on the journey in part because their son, Joaquin, had been an activist on issues ranging from gun violence to discrimination. Next to her on the bus, Sam Schwartz — whose cousin, Alex Schachter, was also killed at Parkland — noted that, in the eight days since the bus tour began on July 3, there had been dozens of incidences in which four or more people had been shot.

The families are now pressing for change from all angles. The Olivers set up booths to promote bulletproof vests to highlight the absurdity of U.S. gun violence and also conducted a sit-in at Congress. At Comic-Con they unveiled a bobblehead doll that curses the National Rifle Association, and they published a children’s board book — “Joaquin’s First School Shooting” — with two holes bored through each page. The Olivers also used artificial intelligence technology to produce a video in which their late son appears with a message about gun violence. In March Mr. Oliver was arrested after he disrupted a committee hearing called by House Republicans in support of Second Amendment rights.

Joaquin, a 17-year-old who scribbled poetry and played basketball, was killed outside his creative writing class at Parkland when a 19-year-old gunman, carrying an AR-15-style rifle, began shooting students in hallways and classrooms.

The shooting inspired surviving Parkland students to lead a “March for Our Lives” campaign to press federal lawmakers for action. The road tour, the Olivers said, was an extension of those efforts.

The day in Texas began in Austin, where the group pleaded with lawmakers to take action. After a three-hour drive to Uvalde, they visited memorials around the city, leaving rocks painted by families in Florida.

Brett Cross, the uncle and guardian of 10-year-old Uziyah Garcia, one of the Uvalde victims, hopped on the bus and helped direct it through town. During the ride, one volunteer edited video clips on a laptop. Another was coordinating an upcoming event in Chicago. Ms. Oliver was sharing photos with supporters back home. The bus went over a bump, and Cameron Kasky, a former Parkland student who helped organize the March for Our Lives, braced a stack of boxes filled with the anti-N.R.A. bobbleheads.

Mr. Cross said he was grateful for the chance to gather.

“It’s family,” Mr. Cross said. “It’s a family that I never wanted. I hate that we know each other this way. But I’m glad that I have them.”

At the event in the park, families coming from Parkland and Santa Fe spoke to the crowd and assembled television cameras, urging those not touched by such a tragedy to also heed their calls. Ms. Hart called on people to vote.

A video screen on the bus flashed images and videos of many children who were killed in Uvalde. Their family members, now joined with the other families, also spoke out. Kim Rubio, the mother of Lexi Rubio, said she wondered why their children’s images were not enough to make necessary changes. Vincent Salazar, the grandfather of Layla Salazar, said children were scared to go to school. Ana Rodriguez, the mother of Maite Rodriguez, pleaded for a ban on assault rifles.

Julissa Cazares Rizo, the aunt of Jacklyn Cazares, another Uvalde victim, warned: “We never thought it would happen to us. Do not think for a second that it will not happen to you.”

Many of the families are in contact nearly every day. Mr. Cross’s wife, Nikki, has grown close with Ms. Hart, seeking advice on how to handle memorials, gifts and requests for information.

“It’s not just that,” Ms. Cross told Ms. Hart. “You helped me with what to expect for the first anniversary.”

On that day in May, Ms. Hart came to Uvalde and stood by Ms. Cross, as she navigated the memorial events. At the last event, well-wishers continued to surround families, and Ms. Hart played a role as intermediary, shepherding Ms. Cross out while handling the various tokens and gifts that others had brought. They have continued to support each other.

“I hate that I know them in the way that I do,” Ms Hart said. “I wish they were cool people that I just came across, you know, like we met in quilting class or scrapbook club.”

At the end of the bus trip event, as the park began clearing, the Olivers and their group packed their belongings again, dismantled the sound system, took down an awning and put all the tools, paint cans and cameras back onto the bus.

Their day had started 15 hours earlier.

Their next stop: El Paso (23 killed in 2019).