Drupal 7 Rules - Set value of multiple value reference field (user reference)
Install next modules:

Conditional Rules 
Rules Bonus Pack 

Create a rule for "Content is viewed".
Set conditions , like "content is type"
Create a loop and set its name, in the data selector choose the field(not the deltas) like [node:field-alumnos-curso]
Create an action inside the loop(do not "set data value"), just select "add item to the list" and the value will be the list name or loop name.
Remember create a action "Save an entity".