I have 3 suggestions for this plugin.
1) When I tried to send blank invite message, this process working. A warning message could be good for this.
2) Register url could be changed
You can create your account here:
We are looking forward to seeing you on the site.
This plugin only compatible with multisite but url, not for multisite. So http://sitename.com/wp-signup.php would be better. (I know wp-login.php redirect to signup page)
3) I tried to contact importer. (this is very useful feature +1 from me) I guess this feauture need to access incsub servers. I entered api key ($invite_incsub_authcode ) but not worked for me.