Looks like "Invite Users" is handled by a plugin, and therefore can't be customized the way "Create Users" can.
BTW, if you're happy with using "Create Users" but don't understand Velocity Macro files (I don't!), you can customize part of the email by editing the "template.jira.user.created.intro" property in the following file (and then restarting JIRA):
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
We're bringing product updates and pro tips on teamwork to ten cities around the world.Save your spot