Adding items to node / user references with Rules?

Is it possible with Rules to add items to a node or user reference?

For example, I have a Project node with a user reference field called Members - many users can belong to this single Project. If I want to add another Member based on some event, can I do it with Rules?

I've tried 'set a value', add item to list' actions, but I can't seem to access my user reference field. I'm already able to find the node I need, I just don't know how to add the new user to the user reference field.

This query is from a previous problem I've been trying to solve, it's a 'new' question resulting from the original query which I thought deserved its own discussion, hope this is OK. Thanks!