This disclosure provides systems and methods for lockout-tagout procedures and systems supporting the same, including, without limitation, locking devices that can store and be locked simultaneously by multiple keyholders. In various embodiments, a single user can add multiple keyholders to a locking device. The user may not be able to remove anyone as a keyholder except himself or herself. Thus, the locking device may remain in a locked state until each of the added keyholders applies their digital key, password, PIN, and/or other personal identifying information to unlock the locking device by removing themselves as a keyholder.