Hello,
The issue is that when you add a user to a group, you actually modify the group, not the user. In particular, you need to add the user's Distinguished Name (DN) to the Member property of the group. The Member Of property is just a back link in AD.
Since on the Create User Form the new user account is not actually created in AD yet, the user doesn't have a DN, and there is nothing to add to the Member property of a group.
We have a similar request in our product backlog. In the future, we'll think on some sort of a way of specifying the groups a new user needs to be added to. Currently, you can, for example, add new users to appropriate groups automatically. For examples on how to do this, see Automatically Add Users to Groups by Department and Automatically Change Group Membership Using Scripts.
Alternatively, if the above methods don't work for you, we can suggest the following workaround. On the Create User Form, you can make available a certain AD attribute of a user account that supports the DN syntax and allows multiple values. If you use an attribute that supports the DN syntax, users will be able to click on a Browse button associated with the attribute and select an object directly in AD. Also, you can configure the attribute to show groups only. Thus, users will be able to click on the Browse button and select a necessary group. If the attribute supports multiple values, they will be able to add multiple groups. For this purpose, we suggest using attribute See Also (LDAP name seeAlso), if you don't use it for other purposes.
Then, a Business Rule triggered after creating a user will add the new user account to the groups whose DNs are specified via the attribute. The Business Rule will need to run a PowerShell script. For information on how to run a PowerShell script automatically after creating a user, see the following tutorial: http://www.adaxes.com/tutorials_Automat ... ngUser.htm. Managing group membership with the help of PowerShell scripts is described in the following tutorial: http://www.adaxes.com/tutorials_Automat ... cripts.htm. If you need, we can help you with the actual script.