List() works on "any related field, repeating field, or set of non-repeating fields; or an expression that returns (same).
So if you want a list of the values of a field in the current table, you must create another table occurrence of it and you can join using the X (cartesian operator); joining any two fields except calculations, globals or containers and they don't have to match. This will allow ALL records to be related and will show field values from ALL records in your calculation. Then the calculation would be (result is text):
List ( yourTable selfJoin::fieldName)
But this will also give you duplicate values. Is this what you wish or do you want a unique set returned?
I have a similar issue where I'm trying to get a field to show a list of all the unique IDs after a find. Suggestions?! Thanks!