Additional features, settings, or issues not covered by specific Microsoft Teams categories
Dear MM Allegro,
Thanks for your feedback!
For confirm the situation better, I just did a test follow the steps I provided above, and my collage (as guest) was invited to join Live event as presenter successfully.
Therefore, according to the description you provided, I think the issue is the permissions of the guest, please check the “Guest access” in Teams admin center as below screenshot. (Be sure all items were opened for guest)
In addition, if the permission is ok, I sincerely hope that there are some screenshots of operation which you could provide. So that I can do more test or research against your situation.
Thanks,
Linda