Bees do not urinate because they have to hold onto as much water in their bodies as possible. Instead, they release liquid waste in the form of uric acid.
Uric acid contains very little water, so it is safe for the bee to get rid of. Insects such as bees produce waste in an organ called Malpighian tubules, which are the insect version of kidneys. Ammonia and uric acid travel to the insect's gut where it is mixed with other waste products before being expelled from the body.
The amount of uric acid a bee expels is very small, so it is usually hard to see. The uric acid from a bee is white and looks like a bird feces.