GRAPHISOFT has recently released Updates for ARCHICAD 18 and 19. These Updates make these 2 versions compatible with the latest MAC OSX 10.11 El Capitan
For more information: http://helpcenter.graphisoft.com/technotes/operating-systems/mac-os-x-10-11-el-capitan-beta/
Please note: If you are using ARCHICAD versions 17 and older, 17 SOLO and older, SE 2014 and older: Do NOT upgrade your MAC OS to MAC OS X 10.11. These versions will not be supported on MAC OS 10.11.