External Direct Messages are private channels where one person is a member of the organization and one is not.
To start an External DM:
- Click the blue New+ button at the top-left corner of the screen and select Direct Message
- At the bottom-left corner of the pop-up, click Invite by Email
- Enter the email address and name of the person you wish to invite
- Click Send Invite
- Confirm that you wish to start a DM with someone outside your organization