The Welcome Message is sent automatically to all course participants and anyone that is added to the class after the "Start Sending" date.
By default, the Welcome Message will only send out to students when 1) the course site is made available AND 2) the "Start Sending" date has arrived. You can override the course availability requirement, however, by checking "Bypass Availability" at the bottom of the Welcome Message page.
After its initial release, there may be a delay before it is received by students. This delay could be up to an hour.
For "project sites", the welcome message will not be disseminated after the "Stop Sending" date has arrived. Regular academic sites do not have a "Stop Sending" date.
At the top of the Welcome Message page, it will indicate how many people received your message.