A group of cells that secretes substances into the blood or directly into an organ

Well, e. g. Pancreas (an hormonal glandule) secretes insulin into the blood stream

A group of cells that secretes substances into the blood or directly into an organ is known as the endocrine glands.

Explanation:

Endocrine glands can also be referred as ductless glands as they lack in ducts for targeted secretions. The pituitary glands, adrenal, thyroid, parathyroid and pineal glands are the endocrine glands in our body.

They release their chemical produce (called hormones) are directly reaches into the bloodstream which regulates various processes happening in human body. The Hormones when secreted in the interstitial fluid mixes with the blood and then reaches to various organs to perform it's function.