Nelle faq ho trovato questo, ma non ho capito cosa fare:

** The user promotion (putting user into admin group) worked, but I cannot put user back into other groups in windows! **

This is known to happen sometimes. Try the local user part of "computer management" in "administrative tools", it is more detailed than the stupid control panel applet. But that may not work, either. Sorry, have no other known workarond. I told you it was experimental!