Thanks, that's what I was figuring out.
I'm not able to create anything with PHP so I have to adapt.
What was interesting with the options is that I easily created with PHP Callback a drop-down menu with the different user groups. With the style properties I don't have this possibility and therefore I have to create a numeric field in which the users will have to indicate the usergroup iD with which they want to interact, which is much less user-friendly.