Do i need to register to play minecraft multiplayer? If so then how?

I have downloaded team extreme minecraft version 1.7.9 and i want to play multiplayer so when i connect to any of the server i says that "Failed to connect: invalid session (try restarting the game)". If do i need to register to do so.

Guest

You should have registered to be able to download the game but its on:
www.minecraft.net

SmallerMe

Yes, all legal copies of Minecraft require registration.

Evolving Squid