I am invited as a user to an organization. So I want to accept and fill in my e-mailadres. Then I can’t fill in the main password.
Hi @Piet, welcome to Community! 
You will need to use an existing Bitwarden account to login or Create a new account from the invitation email:
If you have never used Bitwarden before, you will need to click on “Create Account” and then enter a new master password.
I hope this helps, but if not, feel free to explain your error further or contact our support team for more help!