In Webex Teams, you can invite others to join your universal channel by sharing the link that the bot generates after you create the channel.
How to retrieve the invite link
Open the direct message you have with the Mio Universal Channels bot and send the command create. Name and create your universal channel here.
After creating your universal channel, the bot will give you a message with the link you can send to your external contacts.
Copy the link and send it to anyone you want to invite. They can join the channel through that link regardless if they're using Microsoft Teams or Slack.
Do Webex Teams users need to join from that link as well?
If your colleagues are in the same Webex Teams organization as you, you can invite them to your universal channel as you normally would in a regular space.
In your People tab, add colleagues by name or email.