I'm experiencing the same issue reported above on a different theme when it comes to the user registration add-on.
On this new theme, running on WP 3.1.3, I created the same registration form used on a previous theme, used the simple field, named it Username, mapped it under the Registration form, only this one is not pushing the User registration into the WP users section, they show up under Entries of GF.
I followed the test steps to isolate the issue: Activated Twenty theme, at first, tried deactivating the Member plugin and reactivating it thinking it can be the cause of the issue, then deactivated all non-GF related plugins and tried again, even deactivated GF plugin and reactivated it thinking it might reset some misconfigured settings, yet users are still showing under Entries, to no avail.
Went back on the other site where everything is working fine and did another live test, the user registration is pushing the entry under WP's users which is working properly, so its not a GF version conflict.
When viewing the Entries in the backend, the User column field is empty, no names showing up.
One other thing I noticed between one install and the other, the one that is working correctly, I can see an additional field labeled: Set As Post Author (?) under the User Registration Settings.
If you need admin level access to the wp panel, let me know, I'll pm you the website link along with the admin credentials.
Posted 8 years ago on Thursday June 30, 2011 | Permalink