To send a message to all your clients, you will need to add them all in the same group and use the "chat" bubble from this group.
N.B.: Even if they are added to a group, clients keep their individual files.
Groups are always confidential. Members of a group do not see who the other members of that group are.
The only way for them to know they are in a group is if you communicate with all members using messaging. When a client replies to this message, everyone else in the group will see their response. This message can only be sent by one of the active participants of the group.
For help creating a group, click here.
For help with adding clients to a group, click here.