Arcanum's main module -- which is what's usually being discussed when people talk about Arcanum -- is single-player only, for a variety of reasons. Multiplayer uses different modules, which are basically shorter, much less complex standalone scenarios, set in the same universe and using the same mechanics; I believe GOG's release initially only contains the Vormantown module for this purpose, though several other ones exist (and can be downloaded here, if you're interested).