The Welcome Message will only send out to students when you make the course site available* AND the Start sending date has arrived.
After this, there will likely be a brief delay before it sends to students (these messages are sent out hourly at 46 minutes past the hour). It will then send automatically to all course participants and anyone that is added to the class after that date.
It will not send to anyone that is added after the Stop sending date has arrived.
At the top of the Welcome Message page, it will indicate how many people received your message.
* Note: you can override the course availability to students requirement at the bottom of the Welcome Message page by checking "Bypass Availability."