I think there's also language about cardmember must stay and must pay with AmEx card in his/her own name. To me that would mean that your room plus two others get the benefits, unless you somehow were occupying two rooms yourself, in which case four additional rooms (rather than yours plus two) could get the benefits at a huge additional cost by using both cards. I don't see how you could get your room plus five others with benefits.

Doesn't anyone else in your group have a Platinum charge card?

Remember that Centurion FHR benefits can be different and better than (business or personal) Platinum FHR.