What is an email group?
An email group (also known as a mailing list) allows you to invite somebody to a channel even though they don’t use Slack. This way, any message posted in this channel will be sent to all of the channel's recipients, whether they are Slack or email members. Example uses include email@example.com to work in Slack with clients who only use email.
Email groups have what’s known as a list behavior: When someone replies, the email necessarily goes to all the list’s recipients, it’s not possible to edit the ‘To’ field to remove a particular recipient. Email groups are the closest thing to a group chat conversation in the email world.