We can't recommend a management strategy without seeing the insects or symptoms.  Write us back, but this time include multiple, clear photos (top and bottom of affected leaves).  It helps focus the camera and gives us a sense of scale if you hold the plant leaf with your hand behind it.

If the "glistening, clear wetness" is sticky, too, it is probably what is called "honeydew," the excrement of aphids and scale insects and suggests that one of those pests might be what you have.  Compare your insects to the aphids described here:

https://extension.colostate.edu/topic-areas/insects/aphids-on-shade-trees-and-ornamentals-5-511/

The green insects are aphids, a very common insect pest. They multiply quickly so the numbers can be alarming, but they also vanish fairly quickly as the season progresses.  The least toxic methods for getting ridding of them are simply wiping them off with your fingers or hosing them off with a squirt from the hose.  Washing your fruit before you eat it will take care of any clinging to the fruits.

Curl-leaf aphids refers to aphids whose feeding causes leaves to curl around them into a protective cocoon which makes it harder for predators and homeowners to make contact with fingers, water, or pesticides.  For these types, you can opt for a pesticide with a systemic mode of action, meaning it travels through the vascular system of the plant and is ingested by the pests when they feed.  These products are effective but be aware that they kill non-target insects feeding on the plants, too so they kill the good with the bad.  Refer to the fact sheet on Aphids that I sent you yesterday for additional information.

I cannot see the insects on the Ninebark - it's too dark. Aphids come in different colors, so they might also be aphids but if you want us to try to identify them, shake some of them onto a white paper background, include a coin for scale, and send us that photo.