Team Chat Management

Welcome to the Zoom Community, @gracelancer.

I can't really tell if you did anything wrong... but had you looked through the steps of How to add an external contact in this Zoom Support article?

https://support.zoom.us/hc/en-us/articles/360056768612-Getting-started-with-Zoom-Team-Chat

If you've followed those recommendations and still are unsuccessful, please grab a screen shot of any error message (please redact anyone's personal info) and I'll see what I can do to help you.

Hello gracelancer,

This is likely a setting that you need to enable when creating or editing the channel.  So, you have already added them as a contact- good.  Now, when you create the channel you can add members, but there is an "Advanced" options underneath the members box... expand that option.  The default option is Internal members only.  Change that to allow external members into that channel.  Set your permissions on who can add external members, who can post messages and other features in the Advanced menu. 
Now, go back to the members box to add the external members.  
Hope this was your solution.