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The registration part is really just to get access to the bug-report/support sub-forum. Not needed for MP.

I guess you get "Authentication Failed" ?

Random joining a Magicka game isn't working that great (mostly). I guess there just are to many things that can go wrong.
Host need to have a fairly good upload speed for the internet connection. ( and you don't know if that's the case, joining any server out there )
Ping need to be quite low to avoid lag and verification issues.
Many games are locked (passworded) so those you cant "just" join. You will need the psw.
The game need to be in the lobby, you can't join a game in progress.
So ... Joining just "any" game out there will many times fail, usually with verification failing.

Best way (normally) is to join a group, and get invited through steam.
The host has to consider upload speed and filter settings. ALSO, both host and you have to open a few ports! Like this: http://forums.steampowered.com/forums/showthread.php?t=1739191