Hello,
That's not possible because when you are adding a member to a group, you don't modify the member. Instead, you modify the Member property of the group by adding the new member's Distinguished Name (DN) to the property. The Member Of property is just a backlink to all the Member properties of all the groups that an object is a member of. Since at the time a new user is created (from scratch or by copying another user), the new user is not yet an actual object in your AD, it doesn't yet have any DN, so there's nothing to store in the Member property of the group.
As a workaround, you can add a simple text field that allows users to input the names of all the groups that the user should be added to. It won't be possible to browse for the groups or select them in AD, but if your operators know the names of the groups the user should be added to, they can just input the names as a comma-separated list. Then, when a user gets created, a Business Rule triggered after creating a user will add the new user to the groups specified in the text field.
but it would be great to get a warning or a message in the customization of the web page forms that should say "you cannot add "membre of" properties in a create forms
Thanks for the suggestion, we'll think on that.