Outlook Meeting Forward by someone not invited

I sent a meeting invite to 3 people at my company.  I later received a notification that the meeting invite was forwarded to a person by that same person, even though they were not on the original invite. Typically that forward would have to come from someone invited. How did this person invite themselves to a meeting via a forward from themselves? 

Example: Bob not invited to meeting. I received an outlook notification that "Bob forwarded the meeting to Bob"