Steps to Invite External Attendees to Recurring Events in Microsoft Calendar:

1. Open Outlook calendar and click the plus sign (+) to create a new event.
2. Enter a name, date and time, and other relevant details for your event.
3. You can either enter attendees' email addresses in the 'Invite Attendees' box or enter their name if they're in your contact list.
4. Choose either the 'This is a Recurring Series' option or the 'Make This a Series' option in the 'Recurrence' box that appears when you've selected the invitees.
5. Click the 'Send' button to send your invitation with the recurring series.