From my experience this is all depends on people in question. We've been experimenting in my circle of friends of about 20-30 people over these years with trying different solutions for event invitations : email, g+ events, FB events, SMS, various chat services.

The outcome was that some people simply aren't reliable to follow up if they're fine with the event terms or not, no matter what medium. It's not a tech problem, it's a people problem and everyone's responsible.