There might be a stable invite sytem available for free for our forum software but to install it we would have to upgrade yet again. That might mean some upheaval and downtime and I'm not entirely sure if it would be worth it? Upgrading should really be done at some point as the later versions of SMF have better security than some of the other release candidates, so it may simply be a matter of time before we have to endure a change anyhow.
I thought I'd post this now as a little forewarning.