Hi @lmaza,
Welcome to the Community.
Microsoft Teams doesn't support bulk uploads of webinar registrants from a CSV file. As a workaround, you can use Zapier to automate the process by triggering an email invitation with the webinar link whenever a new row is added to a Google Sheets registration list. While this method is effective, it may not be ideal for all needs, so it is recommended that you reach out to Microsoft Teams support for more comprehensive solutions.
I hope this helps. If you have any other questions, please don't hesitate to ask.