In Slack, you can invite others to join your universal channel by sharing the link that the bot generates after you create the channel.
For steps on how to create a universal channel, follow this article first.
Retrieving the invite link & inviting by email
Find the Mio app and send "/mio menu".
Select My Conversations.
Find your existing universal channel from the drop-down list.
The Mio app will show your channel details.
Copy and share this link with any external contact. You can also send a direct email invite to your contact's inbox without leaving Slack.
Your external contacts will be asked to install the app on their own chat platform, whether it's Slack, Microsoft Teams, or Webex.
Inviting contacts directly from a universal channel
Type "/mio invite" directly in any universal channel (not with the app).
Note: This slash command will only work in existing universal channels in Slack.
Inviting local Slack users from the same workspace
If you want to invite other Slack users in your same workspace to a universal channel, you can invite them how you normally would in Slack.
Type /invite in your channel and @mention your colleague, OR
In the right sidebar of your channel under About this channel, click on the Members tab. Then click the Add People button and select Add to channel. Press Continue.
View all the members in any universal channel.