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):
Hello, Community! My name is Gosia and I'm a Product Manager on Jira Server and Data Center here at Atlassian. Since 2002 when we launched our public issue tracker, jira.atlass...
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