Please make sure that your colleagues have a user account on the same server on which your project was created.
If your project is on a university server, they need an account on that server.
If your project is on www.soscisurvey.de, they need to register an account on www.soscisurvey.de.
Different servers are working completed separate. If you're sure, your project and the accounts are on the same server, please send an email with the project's URL and the email address of one colleage to info@soscisurvey.de, so we can take a look.