In fact here is the InfoObject that is being sent, and you can see that the userGroup is specified.
Yet the user A500035 is created without the group:
<inf:InfoObject xsi:type="user:User"
xmlns:user="http://enterprise.businessobjects.com/3.0/user"
xmlns:inf="http://enterprise.businessobjects.com/3.0/infoobject"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<inf:Name>A500034</inf:Name>
<inf:Description>Best User</inf:Description>
<inf:CUID>AZvcXKA0fiBAt_OlnWorNhQ</inf:CUID>
<inf:ParentCUID>AXhmigik4CBKra9ZYzR2ezE</inf:ParentCUID>
<user:EmailAddress>A500034@add_user_sample.com</user:EmailAddress>
<user:FullName>A500034</user:FullName>
<user:Groups>
<user:UserGroupIDs>AS3BN0tYDc5DtiS5dx5ehNg</user:UserGroupIDs>
</user:Groups>
<user:PasswordExpiryAllowed>true</user:PasswordExpiryAllowed>
<user:PasswordToChangeAtNextLogon>true</user:PasswordToChangeAtNextLogon>
<user:Password>password</user:Password>
<user:NamedUser>true</user:NamedUser>
</inf:InfoObject>