The "pre-existing flying site" is the one that was established first. The newer club must prove there is no interference with the established site, or get the pre-existing club to sign a frequency sharing agreement. In our case, the new 2.4GHz club's charter (1.4 miles away) was held back until president of the pre-existing club (me) told the AMA rep that we understood and agreed the new club would be 2.4GHz only.