_setPropertyForMember(property, value, memberId, unset = false) {
    const listOfMembers = this.members();
    let finder = null;
    if (memberId) {
      finder = member => member.id === memberId;
    } else {
      finder = member => member.selected;
    }
    listOfMembers.forEach(member => {
      if (finder(member)) {
        if (value !== null) {
          member[property] = value;
        } else {
          delete member[property];
        }
        if (!unset) {
          // we shortcut if we don't care about unsetting the others
          return false;
        }
      } else if (unset) {
        delete member[property];
      }
      return true;
    });
    // Session.get gives us a copy, we have to set it back so it sticks
    this._refreshMembers(listOfMembers);
  }
Will look into the list of members to import for the specified memberId,
then set its property to the supplied value.
If unset is true, it will remove the property from the rest of the list as well.

use:
- memberId = null to use selected member
- value = null to unset a property
- unset = true to ensure property is only set on 1 member at a time