Reply To: BA Tier benefits for travelling companion

[yonasl]

Check-in on Business counter: yes for all
Seat selection: yes for all (can select any seat other than emergency on LH)
Luggage: Yes for all (all get +1 on normal fare)
Security priority lane: 1 guest
Lounge: 1 guest

This can change depending on airline:
For instance on Iberia, as silver you can buy the cheapest ticket and check-in your luggage for free (something you cannot do on BA where the cheapest ticket does not include luggage). But your companions cannot (this is not always enforced tbh).
On Cathay only those with status can select a free seat, so you would be able to but she wouldn’t.