To send a message/file to many people at the same time, you must create a room and invite in all the people you wish to send the file/message to.
This could be a room where all the people in the same department are invited in.

When you write a message in this room, everyone who is invited will be able to read it.

You can also forward existing messages to this room.

Se also
Invite users in to a room


Send to many room shares