How can I play games online with friends if I downloaded the game using a torrent?
Moral aspects aside, Borderlands 2 is on Steam, meaning that any online play outside of this system is impossible. If you crack the game you might be able to play the single-player campaign (minus all the things such as achievements and the like), but that's about it.Like all the other people here said, wait for sales or check out marketplaces like G2A where you can get your copy for just a couple of bucks. Or you might even be able to grab it for free if there's a giveaway or at least a free weekend. In case of the latter, there was one in the past, after which you could purchase the game with a really sweet discount on Steam.
How do I do peer to peer with Steam?
P2P/Peer to Peer = Torrents = Users download and upload files to each other. However,Steam offers no such services to its users & is strictly based on a Client-Server networking model. The games are downloaded directly from the distributor’s servers & that’s that. If you’re experiencing slower download speeds,then either upgrade your broadband plan or try downloading from the local servers (Steam>Settings>Downloads>Download Region).