I had that problem (neither companions' portrait showed up, though their dialog changed as if they joined the party) in 1.0.93/95, but a new game in 1.0.102 (after renaming the '..\Documents\Larian Studios\Divinity Original Sin' folder) worked fine.
The D:OS folder in My Documents contains the saved games, configuration files and a level cache folder. Deleting or renaming it will get the game to recreate it on startup; playing the game from a different Windows user account would effectively do the same thing.
