This is a restriction Facebook places on all invitations. It's not something we can control. Sorry it's not easier.
wow i just had the same question
its Facebook not Ning.....I tell you that Zuckerberg is really something! I think on the back end of things Facebook limits a lot of what us small networks can share and link to their site and I feel its to oversee what may actually be next! I've programmed my own invite button on my site that lets me invite 50 friends at a time...go to my network and check it out. You'll find it on the home page under a module I programmed "network tools" click the icon that looks like a little facebook address book
Wow had the same question too. but I understand facebook has their own restrictions to stop spammers.