Remember that you can create a global function that can be called whenever you need in the back-end.

In this particular case the members of a group must be pulled in order to filter a User reference field in the form by a Group reference field in the same form. This is a typical example of Assignment Group and Assigned To inter-dependency. When you choose a group in the Assignment Group field, the Assigned To field should only display user members of the group selected.

With a simple global business rule you can filter the Assigned To field easily:

function getGroupMembers(fieldName) {
    var groupValue = current.getValue(fieldName);

    if (gs.nil(groupValue)) {
        return "active=true^EQ";
    }

    var member = new GlideRecord("sys_user_grmember");
    member.addQuery("group", groupValue);
    member.query();

    var list = [];
    while (member.next()) {
        list.push(member.getValue("user"));
    }
    return "sys_idIN" + list.toString();
}

Here an example:

Easy, clean and practical in the data dictionary of the Assigned To field: