8.1 How can I start or stop getting the list posts grouped into one big email? (digest option)

Groups of posts are called "digests" in Mailman. Rather than get messages one at a time, you can get messages grouped together. On a moderately busy list, this typically means you get one email per day, although it may be more or less frequent depending upon the list.

You may also want to look at Section 8.2 which discusses MIME and plain text digests.

To turn digest mode on or off using the web interface,

Log in to your member options page. (See Section 3.1 for more details on how to do this.)
Look for the section marked "Set Digest Mode."
Set it to "On" to receive messages bundled together in digests. Set it to "Off" to receive posts separately.

To turn digest mode on or off using the email interface,

Send a mail to LISTNAME-request@DOMAIN with the command set digest plain or set digest mime or set digest off.
Commands can appear in either the body or the subject of the message. (See Section 3.2 for more information about sending mail commands.)
Set it to "off" if you wish to receive individual posts separately, and to "plain" or "mime" to receive posts grouped into one large mail. See Section 8.2 for more information on plain versus MIME digests.