Discord uses an expiring URL per lobby, I think, so you'll need to ask for one.
Skype is Skype so I think you need to be contacts and then invited.
TeamSpeak/Mumble are server based so you need the specific community's info and password if required.
Of course, obviously have the programs installed, though I think Discord can be used on a browser (I've only ever used the mobile app).
Also also don't be a creeper and join random servers just because their info is available. A lot of communities have their own game specific channels and I don't know if there's a generic NeoGAF one.