I say you go for your grandma and only that.
I am a Christian, and if someone were to invite me to some Atheist seminar I would go, but not just for them. I would go to discover what others have to say, because it is a good thing to see all perspectives this world has to offer. In fact, an atheist speaker may even shed some light about some worldly things for me.
It is a horrible thing to pull yourself into a protective shell and not listen to others or explore the truth.