Balloon-assisted jumping

Some friends of mine are obsessed with the idea of buying weather
balloons, filling them with helium, attaching them to a harness (with
probably about -3 to -5 lbs of net lift) and jumping up into the air.
If anyone has info on both the feasibility (with implicit danger) and
cost of such an excursion, you input would be most appreciated.

In addition, if this post better fits another group, please feel free
to redirect me (I have already posted to .balloon and .misc). Thank
you.

JumpingMan