To keep informed. To share information with them.

to keep people up to date with information or a situation

To keep someone in the loop simply means to keep everyone informed about any changes to what has already been discussed and planned. An example the manager at a meeting could say to the attendees that he called the meeting just to keep everyone in the loop about changes that were happening within the department.

It means to keep in touch or share information with others.

Kept in the loop means everyone would should be sharing the same information together and know what's happening