3. If at all possible, when sending to a group of people, send to them in the BCC line: Blind Carbon Copy. [Those old enough will remember that letters would be sent to people with 'blind carbon copies' sent to others through using carbon copy paper - ancient technology!] That means that people in the BCC line don't know who else got the email. It also means that hitting REPLY shouldn't spread it beyond who needs to be replied to [the sender].
4. When you send email to a group of people, everyone on the TO or CC line of the email sees, can copy, can reply to, can use, every other person's email address even if that person would prefer not to have it spread so wide. Everyone placed in the BCC line is [as much as possible] shielded as from unnecessary responses or from having their email addresses sent onto others.