If I buy one license of Dungeon Defenders from the current Humble Bundle can I play it co-operatively with my family on two or even better three PCs in the same LAN? I've heard that it supports guest play.
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.