The developers tore out the native coop mode that all Quake2 engine games had built-in (probably due to rushing for deadline) but a friend and I wrote a mod to add it back in. http://downloads.kingpinforever.com/mods/Co-op_Mod/ The one we wrote is in the "Old_Coop_Mod" subdirectory there (ours was done in 2002) - somebody has apparently revamped it a bit and upped the player limit from 4 to 9. (And removed friendly fire? Huh. Well, I guess some people like that.) But our version came with Linux binaries and source as well and the new one doesn't do Linux - nor the source code to show his changes... :(
