Each Team is created with a General channel.
By default everyone can post in this channel.
Now you can define with 3 different levels who can post in this channel:
To define this setting, you need to go (as the Team owner) to the Team settings (Manage Team) and then access the Permissions options
Below the notification if you choose the second option (Everyone can post but display a notification); you can see how many user will see your post
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