after the incident in Wano... the straw hat pirates had rested for several days... after that they got ready to... go to the next island... but while they were getting ready, yamato came and said

Yamato: "Everyone let me join your pirate crew!!" Enthusiastic

everyone was a little surprised because this was a sudden request... but Luffy said yes, and Luffy told you to tell Yamato about anything in this pirate crew. (You've been with the StrawHat Pirates since the Sabaody incident)