When Giorno joined Passione, he entered into Bucciarati’s team. They were assigned the task of delivering Trish Una to the boss. After discovering that the boss is trying to kill Trish, his daughter, the gang rebelled against the boss. However, despite ultimately killing Diavolo, the boss, many of their members are killed, as Giorno and Mista are the only members of the team that survived.
Correction
Um actually … Fugo is also still alive, though he didn’t join the rebellion against the boss.
You quit the team, you’re no longer a “member of team”.
I thought about that reasoning, but if we go through the dialogue by the boat, we see that neither did Fugo expresses leaving the team (he is only against going on a “suicide mission” against the boss), nor did Bucciarati excommunicated anyone for not embarking on the mission.