Hey Jeff,
Sorry, but right now we don't restrict that ability out of the box. We can restrict it for communities with a system property, but there's nothing built in for groups. One fix that you can try is to customize the /template/invitation/invite.ftl. This is the screen that people see when they click the invite link while in a group. You could put in text warning them to not invite people that aren't in a certain dept of your company because you currently don't have Jive SBS enabled for the others yet. If you would like to go this route, we have a lot of great documentation on theming, some of which I've included here:
Themes Overview
Themes FAQ
Product Documentation
I hope that helps. Let me know if you have any other questions.
Thanks,
Kevin