You can join a channel on Slack by going to your channel settings and selecting the default forum that is available to you. If you are an administrator, you also have the option of modifying the channels that will be available to members of your group.
Steps
Method 1 of 2: Join a channel

Step 1. Open a browser and sign in to Slack
To do this, you should enter the name of your channel.

Step 2. Click on Channels on the left side of your screen
This action will bring up the menu of available channels. You'll find this option just at the bottom of your group name section. You can also use keyboard shortcuts to open this menu:
- Control + ⇧ Shift and press L (PC)
- ⌘ Command + ⇧ Shift and press L (Mac)

Step 3. Review your channel options
In the section Channels you can join, you will see a series of channel names belonging to your team.

Step 4. Click on the channel you want to join
This action will open a channel content preview window, allowing you to make an informed decision before deciding to join the forum.
If you have the choice between several channels, you can click in the sort field to the right of the search bar and which is located at the top of the screen to choose a filter (for example, Creation date)

Step 5. Click Join Channel to join the group
This option is at the bottom of the screen. You also have the option of pressing Return to do this.

Step 6. Enjoy your new channel
If you think you joined the forum by mistake, you can leave it at any time by selecting the gear icon at the top of the screen and clicking Exit # [channel name].
Method 2 of 2: Set the default channels

Step 1. Open a browser and sign in to Slack
If you are the group administrator, you can change its settings to define the default forums that your members can join. You should type in your team's name to sign in to Slack.

Step 2. Click on the name of your group
You will find this in the upper left corner of the screen. When you click on it, a drop-down menu will appear.

Step 3. Click the Workspace Settings option
This will open your group's settings menu, from where you can change the default channel settings.

Step 4. Select Expand in the Default Channels section
This action will allow you to modify the parameters of the default channels.

Step 5. Click in the Find Channels field
A drop-down menu of available channels will appear.

Step 6. Click on the channels you want to set as default
Default groups are added to the list for new members upon joining a team.
- Chain #general is the only one that always remains by default. This means that all of your team members will be automatically added to this channel.

Step 7. Click Save to finish
This action will save the changes. Your default channels have been updated!