The results of a given search URL are constantly changing after every campaign. This means, after sending out 500 invites, the next time you run the campaign, people that were 3rd level connections, may now become 2nd level connections. And people that were 2nd level connections may now be your 1st level connections after accepting your invite.

To read more about this issue read this article: Understanding 1st, 2nd, and 3rd Level Connections

In short, the real question Growth-X asks is: "Are there enough results that were not invited yet, within a given Search URL?".