To join a private channel, you need to be added to the channel by a member of that channel.
To join a public channel:
Select the + symbol at the top of the channel sidebar, then select Browse Channels.
Select Join next to the public channel you want to join.
When you join channels, depending on the channel actions configured, you may see a welcome message, and channels may be added to a category in your channel sidebar automatically. Access Channel Actions from the channel name drop-down menu in the center pane to see what automatic actions have been configured.
Select the channel name at the top of the center pane to access the drop-down menu, then select Leave Channel.
When you leave a private channel, you must be re-added by another channel member to rejoin. You won’t receive mention notifications from a channel if you’re not a member of that channel.