Can I play co-op multiplayer with a guest in Dungeon Defenders?
Each computer requires its own license to play, however if you have controllers you can do a split-screen up to 4 people on a single computer. This means that in order to play with 8 people on your LAN, a minimum of 2 computers is required (2 computers each with 4-person split screen).
If you want each person to have their own keyboard and mouse, each computer MUST have their own licensed copy of the game.