if you have friends who have it on steam you can connect to them quite easily using : direct connect
LAN is by definition...local server, so if you don't do it yourself you can't access it.
trying to access random multiplayer games will just result in you being kicked off instantly (been there done that).
global chat server was disabled by larian studios, if i had to guess i'd say it was to prevent a mass amount of insults flying here and there for no reason.
you can create a multiplayer and invite friends in it (you can invite them directly if they are on GoG or they can rejoin you by using direct connect if they are on steam (which is them putting in your ip address so they can access your game, you can find it in option menu if i recall right).
that's about it. you can look around the forum for ppl who also want to start a multiplayer and begin with them.