I want to ba able to concatenate the values of a field of a related list records into a field on the parent. How can I do this?
The only way to do this is with a trigger that does the concatenation when the child record is inserted, updated, or deleted. This is a very costly trigger to write and you should proceed with caution taking into account the number of records you may be dealing with.
As far as I'm aware there is no "point-and-click" solution to this.
No, you can't do this with a formula. Formulas can only do cross-object references to parent objects, not to child/related list records. The reason is that formulas only support referencing to a single record, and a related list potentially (usually) contain multiple records.
As @Bradley points out, you would require a trigger.
Maybe voting for this idea could help:
I want to be able to concatenate the values of a field in related list, i.e., child records. For example, If I have an account object and a contact reocrd. there is a one to many relationship between the two (one account, many contacts). The child record has a text field "second choice", I want to be able to show on each account record a field that concatenates all the "second choices" for all the contacts associated to the parent.