A Gunter Clan is a group of gunters that work together to try and find the egg. If a gunter in a gunter clan finds the egg, the proceeds are split among the people in the clan. As stated in the book, these people are bound together by a ironclad agreement.
After getting the first key, Wade is invited to join the Ovi-raptors, Clan Destiny, the Key Masters and Team Banzai. These are four of the biggest Gunter clans (in size/members or power).
When IOI sets up the force-field on Ludus the big clans join forces to assault the Sixers and reach the first key. (Ovi-raptors, Clan Destiny, Key Masters and Team Banzai and presumably among those that fight).