POD. She invited you. She's hosting the party. Why would the host expect a guest, any guest, to be in charge of making the reservations unless specifically asked?
Sounds like you handled it well, especially since it is pretty minor in the grand scheme of things. And next time you'll know to ask, "would you like me to make the reservation?" Anyway, that's how I'd ask, as a show of respect and appreciation for her treating everyone. Again, that's for the future as it wouldn't have occurred to me to ask in your original situation.
Just curious, when you arrived at the restaurant with no reservation, were they able to accommodate your group?