Jonny,
Create a Client script like this:
Name : Populate assignment Group
Type : onChange
Field Name : cmdb_ci
Script:
function onChange(control, oldValue, newValue, isLoading, isTemplate) {
if (isLoading || newValue === '')
return;
var ref = g_form.getReference('cmdb_ci');
g_form.setValue('assignmnet_group', ref.support_group); // support_group is the name of the field on cmdb from which you want to get the group
}
