– Is it possible to make a login page with a standard username and password, so there is no registration process but you can give only certain clients a username and password so not everybody can register.
You can turn off the registration process by selecting “hide registration” in the plugin’s options. You would also want to disable WP’s registration.
As to making a “standard” username and password, you can create an account and give the credentials for that account to anyone you want. However, keep in mind it is much easier to control things if you give specific credentials to specific users.
I see where you are going with this and I’ll tell you from experience, people think that they are going to give a set username and password to their 10 different users. Then inevitably, something happens between the site owner and user #10 which means that to remove their access to the site, you have to set a new password and send it to the other 9 people. It’s much easier if you can just shut off that one offending user directly without disturbing anyone else.
– Is it possible to only enter the site with a username and password. So when you go to the url you directly get a login screen like wp-admin but then for clients? So nobody can enter except specific clients?
Isn’t that the same as above?
– Is it possible to manually apporove sign ups, so if someone want to sign up for example they have to fill in a customernumber and then you can set the password and username for them and accept?
You can moderate registrations requiring admin approval. What criteria you use to approve users depends on what you want to do. They’ll select a username at registration and a password will be emailed to them when you approve the account.
