OK, so I added 2 linked fields to the Activity table:
- Fullname via Fullname (from Contacts)
- ContactsEntity via Entity (from Contacts). I called it ContactsEntity because there was already the Entity field I mentioned above.
Anyway, those were available to be chosen and I selected Entity first, but there was still only 1 contact showing for each entity. That's when I realized there was a filter on the dropdown that said "Parentfullname" = empty. Once I removed that, everything was showing.
However, this was all fine in the "New Activity" screen in the Activities tab, but when I used a "New Activity" hyperlink from a Contacts grid in the Edit Customer screen, the contact details were not automatically populated.
So...long story short, instead of investigating that, I left the first column in the dropdown as Name via ContactsName, but I added the Entity field as the second one instead.
I'd prefer the Entity one first, but it's good enough for now.