1
general::
Provided with information and included in a decision-making process. For example, She's new to the board, but be sure to keep her in the loop. This expression uses loop in the sense of "a circle of individuals among whom information or responsibility circulates." The antonym out of the loop, meaning "left out of such a circle," dates from the same period. For example, The chairman was consistently leaving Chris out of the loop. [1970s]
American Heritage Idioms
2
general::
Phrase(s): in the loop
Fig. in the group of persons communicating regularly about a specific plan or project. • I don’t know what’s going on with the Jones deal since I’m not in the loop. • Bob and Jean are in the loop. They can tell you what’s happening.
McGrawhill's American Idioms And Phrasal Verbs