Hi - if you just want to display the full name, you could use an expression box. For the formula, you could select the row from the SharePoint list where the userID field is equal to the value of the userName() formula. If, instead, you have a form field you'd like to set with that info, you can do the same thing in a rule on form load, or on the field you are setting to the userName() function in the rules mentioned in your post above. Does that make sense?