We don't have an explicit feature that only allows users from a certain domain to join. However, you can limit the people who join your universal channel but only sending them invites via email. The links that are sent in email invites are specific to that email address only.
If you want users from any domain to join, you can simply send them the invite URL (app.m.io/uc/acbde).
Install Mio Universal Channels in Microsoft Teams, Slack, or Webex Teams to start chatting with external contacts in minutes.