[keycloak-user] Extending "Add User UI"

Hi, what is the easiest way to extend Keycloak UI? In our scenario the user may have a privilege to manage resources belonging in some group, but not be a member of this group. Our idea how to implement this is to have group selector on "Add User" page, which would either store the data into user attribute or into new JPA entity created for this. The question is, what is the easiest way to extend Keycloak JavaScript. I found in docs how to change the templates, but this is definitely much more in depth. Thanks, Pavel