Private channels in Slack are marked by a "lock" icon
If you would like to send notes to one of these private Slack channels, you will first need to invite the FellowBot into the private Slack channel.
To do this, access this private channel and use the command /invite @Fellow
Once that has been set up, when you go to send the agenda/notes in Fellow (and select the option to send via Slack).
The private channel will appear in the drop down