Official comment
You can't actually change a broadcast that has already been scheduled, but you can cancel it and reschedule a new broadcast with the changes made! Here's a video showing how to cancel a broadcast: http://screencast.com/t/T3JuzZYc
To answer your second question, the broadcast will not be automatically sent to the additional 100 contacts who were added to the group after the broadcast was scheduled.
You can use a Date Sequence instead to send the email to everyone! Add the group to the Date Sequence containing the email. Then, configure a Rule to add all relevant contacts who are added to the Group to the Date Sequence. For example, if your Group is based on the condition "Contact Tags Contains Free Download," your global Rule would fire when contacts are added to the Free Download Tag. The action of the Rule would add the contacts to the Date Sequence, which will send the email at the future time you have scheduled.
