I challenged myself to redo TeamStore.js using Pinia Setup Store and came out with this, if anyone is interested.

import { defineStore } from "pinia";  
import {computed, ref} from 'vue';
export const useTeamStore = defineStore('team' , () => {
  const data = ref({
    name: '',
    spots: 0,
    members: []
  });
  async function fill() {
    const r = await import('@/team.json');
    data.value = r.default;
  }
  function grow() {
    data.value.spots = data.value.spots + 1;
  }
  const remainingSpots = computed(() => 
    data.value.spots - data.value.members.length
  )
  return {data, fill, remainingSpots, grow}
});

All reactive data is in the data key, so I had to change TeamMembers.vue, for example:

<TeamMember v-for="member in team.data.members" ...

Build and Seed a Team Store