You can't invite every single person, every single time, to every single thing.
You've said that the people invited to this barbecue are not your regular circle of friends, so you shouldn't be hurt that you weren't originally invited.
But then the hostess is told that you'd quite like to come, and thinks "Hey, that would be great", and invites you along.
If you snub her invitation (remember, she already knows that you don't have anything else on), it will seem a bit hurtful.
Of course you must go.
But contact the hostess a few days before the barbecue, and say ... "Thanks for inviting me, I'm really looking forward to it, can I bring anything, like a pudding or a salad or something, or shall I just bring some wine?"