Thanks k, that's done the trick!
Perhaps related somehow, I notice that although the info says you can invite by name or username, I get an invalid user message when using their name. If I enter a username then it works fine. Just thought I'd mention it. Awesome plug-in, btw!
Joomla 1.5.25, CB 1.8rc2, CBSubs 1.2.2, GroupJive 2.5rc, Kunena 1.7.2, php 5.2.5