To keep someone in the loop means to provide them with all the necessary information, keep the person/s up to date/posted with what's going.
Mary is new to this City Council. Please, take care of Mary and make sure you keep her in the loop on the board issues.
The FBI investigation is going very well. We have to stick together and keep piling up the evidence against Mr. X. Please, keep me in the loop on the newest developments in this case.