Yes, if the plan is hosted in a Microsoft Team. No, if the plan is a Hub-only plan (i.e. not in a Microsoft Team).
Teams plan:
- An external guest member of your Microsoft Team has access to plans in that Team. See Work with external guests for more information.
Hub-only plan:
- Members of Hub-only plans are added using the University's Global Address List (same as the Address Book in an Outlook email) so you can't add an external person to a Hub-only plan.