I was really lost as to what this was for. I thought it might be web application invites (i.e. to social networks). I didn't know that this was for events/parties/etc.
Maybe you can put an example somewhere or even give them an option to send those emails through you with an example email form clarifying?
I thought that it would be easier (not just for me, but for users as well) to let the users handle the invite delivery (email / sms / twitter etc). If they sent it through a webapp, they would have to worry if the invite was eaten by a spam filter etc.
It's great. But please add more description about how users can use it and screenshots of how it will look to receivers. Then this will be a great tool.
Maybe you can put an example somewhere or even give them an option to send those emails through you with an example email form clarifying?