We are going to use a form for a part of a clinical study and there are only 4 persons that should be able to enter information into this specific form.
I understand that I can choose to share the link to the form with only these appropriate persons but I want to know if there is any way I can password protect the access e.g. after they have clicked on the link, they need to write a password, then be able to fill in the form?
This is not something we currently support but you can add or vote on feature ideas: http://forums.adobe.com/community/formscentral?view=idea
If you need to add a new idea click "Create an idea" under "Actions" in the top right.
For now, create the form in FormsCentral as normal then export as a PDF. Protect the PDF with password security. You will need Adobe Acrobat Pro in order to accomplish this.