The results of a given search URL are constantly changing after every campaign. This means, that 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?".