Adding users to channels in Slack is achieved by way of Slack groups.

1. Use Torii workflows to set up an "Add Slack user to User Groups" action:

2. Set the default channels for your groups in your Slack workspace administration tool.

Sending notifications to Slack channel 

To send notifications to private channels you need to invite the Torii app to the channel.
Tag the app as @Torii, and approve when Slack asks you to invite the app to the channel.
If the app was invited to the private channel, you would be able to select to send notifications to this channel along with other channels.