Since Mio Universal Channels connects other chat apps (Slack and Webex Teams) to Microsoft Teams, only the members native to your organization are shown in the Microsoft Teams app.
Before you begin: Make sure your channel is a universal channel first.
How to view all universal channel members
Go to your universal channel. At the bottom of the channel, find the "· · ·" and select Mio Universal Channels.
You should see a pop-up like this one that lists all the members in that Universal Channel.