Direct messages (DMs) are ad hoc conversations between two or more people. DMs can be one-on-one with another person, or they can include up to 9 people in total (i.e., a group DM). DMs are best for quick conversations that don't need a whole channel to weigh in.
When discussing code, it’s generally best to do so in a channel rather than in a DM. That way the discussion is accessible in the future via a discussion marker in the code that’s visible to everyone in the channel. If the discussion happens in a DM, it doesn’t really contribute to your team’s knowledge base because it will only be visible to a limited audience.
Starting a new direct message
From the channel switcher, click on the “Direct Messages” heading, or the “+” button to its right, to start a new direct message.
Start a new direct message by selecting the people you want to include from the dropdown list, and then clicking Go. You can also select from the list of recent direct messages.
Accessing your direct messages
The channel switcher lists all of your direct messages in the “Direct Messages” section.
If you want to get to a direct message that isn’t displayed in this list (e.g., you closed it at some point by clicking the “x” that appears to its right in the list), you can get back to it by starting a DM with the same group of people. This won’t actually create a new DM, but instead just take you back into the one that already exists.
Membership in a group DM
It’s not possible to add people to an existing DM, or to remove people. You’ll simply need to create a new DM with a revised membership.