I used the web service _vti_bin/UserGroup.asmx?op=AddUserToGroup to add a user to the owner group of a site collection. The user is shown as a member of the group but doesn't have access to the site collection. When I use "check permission" the users isn't shown as a group member. Is there anything more I have to do?
I just found the anwer in the nintex community:
The user has to be given as i:0#.w|domain\username and not as domain\username
