I see there is an incident, but not sure it is related as this has been an issue for quite a while, where other agents has had to assign the requested participants for the affected Agent.
So the problem is after reviewing permissions that I have 2 identical Agents, where one of them can't add new Users as participants, but can see those currently assigned.
I have walked through the permissions with the Permission Helper and found NO indication of missing permissions.
I have tried remove his permissions to the Project and re-apply them, makes no difference.
Also to eleminate cache related issues, I have tried different machines and browsers.