Since Mio connects other chat apps (Slack and Webex) 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, find the · · · and select the Mio app.
You should see a pop-up like this one that lists all the members in that universal channel.
Invite more members to your universal channel.