I see in the FAQ you guys compare yourselves to email. I was thinking your real comparison is the phone. I'm not sure why I would use this service instead of just phoning someone. Perhaps I'm not the target market though. Otherwise, good luck to you. It looks like you put a lot of hard work into this.
Have you ever said to someone "a bunch of us are heading to _____ for _____. You should come."? This is the kind of situation we're targeting. If you can easily inform everyone you want to at once, we've shaved a good bit of time off of your phone calls.
One or two people max is the norm for a casual get together. Again, I think this is where I'm not in the target market... as you get older, your desire for large groups of people wains.
:-)