getMember(memberId = null) {
    const allMembers = this.members();
    let finder = null;
    if (memberId) {
      finder = user => user.id === memberId;
    } else {
      finder = user => user.selected;
    }
    return allMembers.find(finder);
  }
returns the member with specified id,
or the selected member if memberId is not specified