"Add attendees" greyed out in outlook

Hello,

I've been working on creating a booking site for some of our employees to make scheduling with customers easier. And while it is mostly working there is still one pain point we've come across.

Example:

Someone will book a meeting with me through Bookings, and I'll accept it. This meeting will appear in my Outlook calendar and that all works well.

I now realize that I want to add someone else to the meeting. I double click the meeting in Outlook and then go to scheduling assistant. At this point, I will see that add attendees will be greyed out. The only option I would have to invite someone else would be to send someone the invite link, which I don't want to do.

I've also looked at the "New" Outlook, and it seems scheduling assistant is completely gone from the meeting screen there.

I've made sure the Booking service is able to have multiple attendees so that isn't the issue.

Does anyone have anything to fix this?

Thanks