I seen a few pictures and heard from a friend or two .. leaving a cup of water in a growbox would make the air humid because the water evaporates into the air ? Is it true ?
I always heard to not do that though for mold maybe? i cant remember but i remember reading that on here a few weeks ago
Really having proper humidity in a grow tent is most likely the least of your worries. Most homes its very hard to keep humidity up as if you run AC you have very low humidity. The primary method a AC works is to remove humidity out of the air.
I sometimes have a bowl of water in my tent when humidy isn't quite right, just remember to keep it clean as after a week or so the bowl looks yucky and you will probably have to top it up every few days