You can't really use a product outside of it's use case, then complain about it when you reach "outside" it's walls.. I mean you can complain all you want about it, but don't blame it on the product itself. The manual invitation form is clearly designed to discourage mass open ended invitations. A product designed for teams with 500-1000 users in mind, is probably going to get bogged down by 10,000+ users.