’Twas brillig — if brillig means a warm summer afternoon, with partly cloudy skies and gentle waves lapping the shores of Nantucket Sound — when Kelsey Grousbeck and Matt Cosby became husband and wife on State Beach July 15.

The couple chose Camp Jabberwocky’s beach spot for their wedding site, inviting all the campers and counselors to be part of the celebration. Guests sat in wheelchairs and folding chairs, shared blankets on the sand or stood in barefoot groups along the beach as musicians Howe Pearson and David Thompson sang and played love songs on guitar.

A rustic arch of tree branches stood near the water’s edge and a ridge of heaped shells marked each side of a processional path across the sand from the boardwalk, which was lined with stands of blooms and lavender ribbons that floated gently in the light breeze.

As the duo sang Don’t Worry Baby, the red Camp Jabberwocky bus pulled into the parking area with the wedding party aboard. They were singing, too: Chapel of Love, with the refrain changed to Beach of Love.

Jabberwocky campers were more than guests at the Sunday afternoon wedding: They were ushers, the ring bearer and readers in the ceremony, and each of the camp’s eight cabins collaborated to make and decorate a wedding cake to serve the crowd.

Camp Jabberwocky executive director Liza Gallagher served as wedding planner and coordinator, shooting two thumbs high in the air when it was time for the wedding party to begin its procession down the boardwalk as the band played Sea of Love.

The bride is a 13-year Camp Jabberwocky volunteer who started there as a teenager, inspired by her brother Campbell, who lost his eyesight as a toddler. After earning a master’s degree in business administration, she became the camp’s first director of development in 2017.

Mr. Cosby is an accomplished bass guitarist who became a Jabberwocky volunteer as well. He, Mr. Pearson and Mr. Thompson played in the house band for the camp play, Sister Act, which ran for the two nights before Sunday’s wedding.

The newlyweds are both professional photographers who met while studying the craft.

“They both have a way of making people shine without making them look shiny, and I think that’s an incredibly special quality both as photographers and as people,” said camp counselor Mike Léon, in his remarks before performing the wedding ceremony.

The couple paid tribute to their extended camp family with a reading of Lewis Carroll’s 1871 poem Jabberwocky, declaimed with pride and confidence by campers Faith Carter and Beth Ainsworth.

Ms. Ainsworth’s “Callooh, callay!” was clear and joyful, though other campers gave way to tears as the ceremony continued.

Mr. Cosby himself had difficulty making it through the words he had written for his bride, stopping several times to wipe away tears during a tender speech that had guests sighing aloud.

“You know me better than anyone else in this world and somehow you still manage to love me,” Mr. Cosby told her. “You’re the greatest person I have ever known and you deserve to hear the greatest words from me today . . . you make me want to be a better person in every aspect of my life.”

Mr. Cosby also pledged to use proper grammar and punctuation when texting and emailing his wife, and to respect her choice of music on long drives — even though she loves a cappella and “it’s like nails on a chalkboard to me.”

The bride also gave way to emotion as she looked back on the couple’s years of friendship and courtship.

“At first I was nervous to share Camp Jabberwocky with you,” she said, fighting tears. “It was my place and my family. But sharing camp with you was the best decision I’ve ever made in my life.

“You have so much love and genuine care that you have made camp even stronger. You have brought your creativity, your passion, your patience . . . and shared all of it with us, every year.

“You have taught me that sharing adds to what you have and doesn’t subtract.”

After the ceremony, guests mingled over food and drinks as campers and counselors posed for photos under the nuptial arch. Some quickly stripped off their party clothes and splashed into the water.

The couple got a surprise wedding gift when the Vineyard Sound singing group turned up for a mini-concert on the sand. Mr. Cosby smiled without irony as he and his wife took their first dance to an a cappella version of the 1950s calypso Zombie Jamboree.

More dancing and photos followed, and then the newlyweds truly took the plunge, heading hand-in-hand into the water with the bride still holding her bouquet. A mass swim followed as campers, counselors and other guests — many still fully clad — joined the couple in Nantucket Sound.

Drying off with a pair of matching flannel shirts, the couple were serenaded once more by the musical duo before heading into their married life together — but not without one final Jabberwocky nod to wedding tradition: The back of the red bus was now decorated with a row of cans on strings, each can painted with a letter to spell out KELSEY & MATT.