I have installed the register_user plugin, but when entering the users details, the "Save" is still greyed out and does not save and the page stays static.
I have tried both with the password showing and hidden without change.
Current version - stable.

This the error on the page ?
[code]
An invalid form control with name='username' is not focusable. input type=​"text" id=​"username" name=​"username" value size=​"48" required>​
[/code]

This is sadly a bug. But it is fixed in 1.6.5a already.

Thank you, is it an easy one to fix as I don't wish to install unstable version at this stage, can I compare files ?

You can try to use the complete plugin from 1.6.5a, probably that works right away. But I can't say for sure as we didn't test it if any other changes are involved.

Thank you, copied file over and did some edits, works now.