The sensation comes from the Macbook case voltage differing from that of ground, and finding a path to ground through your body. Sometimes, when you feel the sensation coming from the computer, if you lift your feet off the ground the sensation stops. This is because you are no longer a path to ground.

It's totally possible for this path to ground to go through two humans, making it feel like their skin is buzzing! For example, if your friend is in bed using a Macbook, they're insulated from ground and thus at Macbook case voltage. If you stand on the ground beside them and touch them, the current now finds a path to ground through both your bodies!