service cloud.firestore {
      match /databases/{database}/documents {
          match /meetings/{meetingId} {

          function correctUser() {
            return get(/databases/$(database)/documents/access/$(request.auth.token.email)).data.obj[get(/databases/$(database)/documents/meetings/$(meetingId)).data.groupId] == "leader"
          }
     }
}

function correctUser() {
get(/databases/$(database)/documents/meetings/$(meetingId)).data.groupId in get(/databases/$(database)/documents/access/$(request.auth.token.email)).data.list
}