I have created a user reference field on a node which lists (via Views) all the users the node author is related to in some way. If there is a better way to list users on node forms then I'm open to suggestions. In my particular case the field uses the autocomplete widget with the Views list, but a select list widget could also be used.
I would like to be able to execute a Rule on the user that is selected in the autocomplete/list by the node author. At the moment I am using the Button Field to trigger the Rule execution but I'm not sure how to get the Rule to apply to the specific chosen user. Some ideas are flagging the user when they are selected or doing a textual comparison between the autocomplete text and existing usernames. I have also tried the Relation module but still can't figure out how to get the selection recognized in Rules.