Hi Simeon Lodge
Thank you for posting on the Microsoft Community forum.
Based on your description "How to I add an external meeting invitation to MS Teams?" There are two ways to add a meeting to your MS teams.
either by accepting the invitation if you are invited to that meeting or add it manually by using a link of that meeting on any other details you have about that meeting.
- Open the email invitation you received for the meeting.
- you can click "Accept" to add it directly to your Outlook calendar, which will sync with your Teams calendar.
Alternatively, if you have the meeting details (like the meeting link and time), you can manually add it to your Teams calendar:
Copy the link,
Open your team's calendar new meeting
past the link on title and save.
Kindly try to check the following article, and try the troubleshoot provided: Add a meeting link to the Teams Calendar | Microsoft Community Hub
Thank you for your cooperation. stay safe!