We are working on an issue reported by multiple customers in regards to external timeline attendees. Our team is currently working on a fix for this, and will share updates on this thread. Please find further information on issue and temporary fix.
Issue: Customers are unable to add external attendees to timeline activities
Temporary Fix: Save the activity as a draft, and then re-edit the draft. You should then be allowed to add external attendees to the activity and save.
**Update** This is resolved now and a fix has been pushed to production.