The Message users option under Bulk action allows you to send a message to multiple contacts directly from the Contacts page. This helps you communicate with several users at once without opening each chat individually.
Inside the Contacts page.
Select the checkbox next to the contacts you want to message.
Click on Message users from the Bulk action bar.
A side pane will appear where you can type your message in the description box.
After writing the message, click Send message.
Once sent, the message will appear in the Inbox for each selected contact, just like a regular chat message.