Although he tried to attack the Straw Hat Pirates at first, Gaimon ends up making friends with them after telling them about his past, and they tried to help him get the treasure chests he had been wishing for. However, with the chests empty, Luffy asked him to join his crew, but Gaimon declined the invitation to stay with the animals.
Tama and Luffy had a heartfelt relationship early on in the Wano arc and bonded further through their mutual relationship with Ace. There were a lot of parallels between what Ace wanted to do for Tama (and failed) and what Luffy wanted to do for Tama (and succeeded). By the end of the arc he made it a place where she could eat however much she wanted without worry.
After becoming Shinobu’s kunoichi apprentice Tama swears that she will join Luffy’s crew when she can properly use ninjutsu, to which Luffy accepts.
Monkey D. Luffy technically first met Kin’emon after defeating the Punk Hazard dragon. Kin’emon’s legs were stuck to the dragon’s skin, which piqued Luffy’s curiosity. Luffy was so impressed by the pair of talking legs that he wished to have them join his crew, constantly playing “centaur” by sticking them to his back. After the events of Punk Hazard, Luffy let Kin’emon and Momonosuke travel with them to Dressrosa in order to search for their comrade Kanjuro.
Upon realizing they shared desire to defeat Kaidou, Kin’emon requested Luffy to allow the samurai to join their alliance, showing that his trust for the crew had increased. After being defeated by Kaidou, Kin’emon begged Luffy to save Wano in their place, which the young pirate vowed to do as it was his friend’s country.
After Kaidou’s defeat Luffy showed his respect and gratitude when he told Kin’emon he was welcome to join his crew if he ever decides to become a pirate.
Momonosuke has grown quite close to the Straw Hat crew during his journey with them not unlike Vivi before him. By the Zou Arc, he grew to trust them with his life and by the end of Raid on Onigashima, he considers the pirate crew as his closest friends and is immeasurably grateful for helping him save his country. When it came time for the Straw Hats to leave Wano Momonosuke tearfully begged them to stay but he knew they had to go out to sea. Before they left, they made a copy of their Jolly Roger to hang in Wano and told Momonosuke to point to it to let people know that Wano was allied with and under the protection of the Straw Hats.
Bartolomeo is the captain of the Barto Club and the commander of the 2nd division of the Straw Hat Grand Fleet.
Bartolomeo, an original admirer of Luffy from the time of Loguetown and onward, admires the crew in its entirety, saying he followed all of their actions.
What job the Straw Hat Crew actually requires is a Guard who basically protects the ship during their adventure in various islands in the New World. This is emphasised by the fact that in almost every arc, the crew splits up and somebody is left behind to protect the Sunny.
Bartolomeo joining the Straw Hat Pirates would have solved this problem. His Barrier Fruit could create a barrier around Sunny while the crew can go adventuring into the islands.