How does one agency attach itself to another?

Fusion Centers are designed to promote information sharing at the federal level between agencies such as the Federal Bureau of Investigation, the U.S. Department of Homeland Security, the U.S. Department of Justice, and state, local, and tribal law enforcement. As of February 2018, the U.S. Department of Homeland Security recognized 79 fusion centers.[1] Fusion centers may also be affiliated with an Emergency Operations Center that responds in the event of a disaster.

https://en.wikipedia.org/wiki/Fusion_center