What command is used to add a user to an investigation in the War Room of Cortex XSOAR?

The command used to add a user to an investigation in the War Room of Cortex XSOAR is the one that employs the "@" symbol. This notation typically signifies a mention in chat interfaces, allowing for direct engagement with the specified user, facilitating their involvement in the ongoing investigation. By using this command, it efficiently alerts the user about the context of the investigation, enabling them to participate actively and contribute to the collaborative efforts.

In the context of other options, while variations of commands may be conceptually appealing, they either don't align with the standard syntax used within Cortex XSOAR's War Room features or do not accurately perform the intended function of inviting participants to an investigation. Thus, using "@" is the appropriate and recognized method to notify and involve users in the investigative process.