The usergroup is not used when adding a user,
if you look at the XUser you see it's read-only.
The updateUserGroup could be used, but you need to specify all existing users, so that would be a bit tricky...
I think you need to use executeSQLUpdate, and add the association via sql.
You need to look at the datadictionary to see whcih table/fields to use.
hth
Sascha