Although team members should be free to join whichever Slack channels are appropriate for their role and interests, it is a good idea to set a few defaults. Default Slack channels are channels that every team member is automatically a member of as soon as they create an account.
Some good ideas for default channels are things like company announcement channels and general discussion channels, just to keep everyone on the same page. As a Slack admin setting these defaults is easy,
1. Click the arrow next to your Slack team name
2. Select “Team Settings” from the dropdown menu
3. Click “expand” next to where it says “Default Channels”
4. Enter the channels you would like to be your team’s defaults
5. Click “Save Settings”