By default, all the channels, which are added from Manage Channels, are accessible only to the user who has added the channels. If you want other team members to access them you need to share them.
- Click on the Manage Channels.
- Click on the Channel Tab, which you are trying to share.
- Beside the list of channel, Click on the SHARE icon.
- Check the members with whom you want to SHARE the channel.
- Click on SAVE.
*NOTE that only ADMIN will have access to add channels and share them with other team members.