For some reason I remember having first seen it just saying "ad agency"; maybe it changed, maybe I'm crazy, whatever ;P.
Regardless, given that those two categories are included, I'd find it difficult to believe that anyone who reads this website isn't in at least one of them: "advertiser" + "publisher" includes anyone who uses AdSense.
Maybe you should list some kind of "minimum size" to be invited? I might suggest "dollars of ads transferred", as that's probably what you are caring most about (big-ticket partners and clients), and handles all three cases quite cleanly (whereas "impressions" or something only awkwardly approximates that metric and doesn't work well for "agencies" at all).
Regardless, given that those two categories are included, I'd find it difficult to believe that anyone who reads this website isn't in at least one of them: "advertiser" + "publisher" includes anyone who uses AdSense.
Maybe you should list some kind of "minimum size" to be invited? I might suggest "dollars of ads transferred", as that's probably what you are caring most about (big-ticket partners and clients), and handles all three cases quite cleanly (whereas "impressions" or something only awkwardly approximates that metric and doesn't work well for "agencies" at all).