adding value(s) to user reference field

Hi,

I need a hint how to solve this:

In their profile, my users can select other users via a user reference field. By doing this, the selected users get the permission to set a reference to the users profile. This works well and is said here only for explanation. But additionally I want to define a rule, triggered when the node is safed, with the following action:

• all users of a specific role are added to the user reference field of the safed node

Is there a simple solution to do this? Or do I have to use some php? If yes, what would it look like?

Thank you for advice!
Sven