The data I've seen says that having someone draft you is slightly beneficial*. for enclosed HPVs, drafting actually increases drag!
Anyhow, don't do it unless you let the other person know. I once had someone latch onto my wheel without asking so I didn't feel bad when I swerved at the last minute to avoid a load of lose wire fence hanging out into the road and onto which this guy hooked his bars. He went from about 20 to nothing very rapidly. This should have learnt him a lesson!
* to explain why, imagine you've got a circular section thing in wind. The drag will lower if you introduce a triangle very close behind it even if it's not the same object.