I'll try to explain as best as I can. (Testing on locale with the default theme)

I want to password protect my entire gallery but I can't give out the same password manually to every user. I'd rather have a multiple users system where everyone can sign-up by himself, choose his own password and I reserve the right to approve or decline the application.

If I understood correctly this is what the register_user plugin was made for. Now, I'm not entirely sure I know how to use it.

First thing I did was activate the plugin, set a password for the entire gallery and put a call to printRegistrationForm() on index.php for the sake of testing. Nothing showed up except the standard form that asked for the gallery pw. So I tried to put a link to the script like the user guide suggested. I put the html just right after the body tag but again, the hyperlink doesn't show, only the form.

I'm clearly missing something, what do I need to do?

First, if you have password protected your gallery, the script that is loaded when someone not logged in visits is the password.php script. This is where you will have to privide the registration.

Second, the way the register-user is usually used is that a new script (perhaps called register.php) is created with whatever lead in text you desire and the call to printRegistrationForm() This way you can have a public page that links to the registraton "form". Note that you should insure that whatever name you have chosen is excluded from password protection. It would work to put the form in password.php but then everytime someone needed to log in they would be presented with the form. Might be a bit confusing to them.

Note: The Effervescence+ them has implemented the registration as described above.

Oh thank you, I switched to the effervescence theme. When I try to add a new user it tells me:
User Registration
Registration failed.
zenphoto_sendmail failed to send Registration confirmation to one or more recipients.
Same with php mailer, do you know what it could be?

Have you entered an e-mail address?

Yes. Btw, I tried with php mailer too, same problem.
PHPMailer failed to send Registration confirmation. ErrorInfo:Language string failed to load: execute/usr/sbin/sendmail

Does emailing work for you in other places, like for password reset or comment notification?

Mmm, it's a general problem. I can post comments but I don't get any notification, same for password-reset emails.
I know I'm working on locale but it's strange cause I've never had this kind of issue before with older zp installations. I'm sure it was working fine at least till 1.2.6.

Btw, just after the install, I've been redirect to the 'general' tab under options instead to the 'user' tab under admin where you setup the admin name and the other details for the first time. Don't know if the problem it's related but I had to struggle before it let me set the password.

The problems are not likely related. As to the error message, that is from the PHPMailer. Unfortunately, I do not know what it means. It seems to have something to do with language translataions. Is your site not in English? Probably if we can figure out this one we will get the real error message.

If you are updating a current install you do not get directed to the page to establish an admin user as there already is one.

I should post here everytime so problems can solve by themselves. I don't know what was wrong with it, after several installs it seems to be fine now. Great.
Btw, my site is in English.
Thanks a lot for the help anyway.

Please excuse my ignorance, I am a new user and not a developer. acrylian - you asked if an email address has been entered...where would this be entered?
Thanks,