I am having difficulty validating an addition to a repeating list.
As you can see from the below picture I want to add a staff member to a course list, but i only want the ability to add as many staff as permitted by another field.
If I understand your image properly - in Only when don’t do joins.
Rather think of something like:
Repeating group with participants items : count <
Search for course organisation’s [in constraints set course = dropbox course value and organisation = dropbox organisation value] max participants